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",[948,1004,969],{},[938,1006,1007,1008,1010,1011,1014,1015,996],{},"函数返回值类型期望是 ",[948,1009,973],{},"，而 ",[948,1012,1013],{},"return"," 语句中的表达式是 ",[948,1016,969],{},[938,1018,1019,1020,1022,1023,996],{},"某个运算符期待 ",[948,1021,973],{}," 类型操作数，但实际给出的操作数却是 ",[948,1024,969],{},[938,1026,1027,1028,1031,1032,1035,1036,1039,1040,1042,1043,1046],{},"在 ",[948,1029,1030],{},"if"," ",[948,1033,1034],{},"while"," 和 ",[948,1037,1038],{},"for"," 的条件表达式中，可以将 ",[948,1041,969],{}," 类型转换到 ",[948,1044,1045],{},"bool","。",[928,1048,1049],{"id":1049},"从类类型转换到别的类型",[918,1051,1052,1053,1055,1056,1058,1059,1061],{},"假设我们要构造从 ",[948,1054,969],{}," 到 ",[948,1057,973],{}," 类型的类型转换，而 ",[948,1060,969],{}," 是类（也就是我们可以“控制”的），那么就可以定义这样一个运算符重载：",[1063,1064,1069],"pre",{"className":1065,"code":1066,"language":1067,"meta":1068,"style":1068},"language-cpp shiki shiki-themes material-theme-lighter material-theme material-theme-palenight","class A {\npublic:\n    operator B() {\n        \u002F\u002F 语句 ...\n        return \u002F* B 类型的值 *\u002F;\n    }\n};\n","cpp","",[948,1070,1071,1088,1097,1112,1119,1132,1138],{"__ignoreMap":1068},[1072,1073,1076,1080,1084],"span",{"class":1074,"line":1075},"line",1,[1072,1077,1079],{"class":1078},"spNyl","class",[1072,1081,1083],{"class":1082},"sBMFI"," A",[1072,1085,1087],{"class":1086},"sMK4o"," {\n",[1072,1089,1091,1094],{"class":1074,"line":1090},2,[1072,1092,1093],{"class":1078},"public",[1072,1095,1096],{"class":1086},":\n",[1072,1098,1100,1104,1107,1110],{"class":1074,"line":1099},3,[1072,1101,1103],{"class":1102},"sbssI","    operator",[1072,1105,1106],{"class":1082}," B",[1072,1108,1109],{"class":1086},"()",[1072,1111,1087],{"class":1086},[1072,1113,1115],{"class":1074,"line":1114},4,[1072,1116,1118],{"class":1117},"sHwdD","        \u002F\u002F 语句 ...\n",[1072,1120,1122,1126,1129],{"class":1074,"line":1121},5,[1072,1123,1125],{"class":1124},"s7zQu","        return",[1072,1127,1128],{"class":1117}," \u002F* B 类型的值 *\u002F",[1072,1130,1131],{"class":1086},";\n",[1072,1133,1135],{"class":1074,"line":1134},6,[1072,1136,1137],{"class":1086},"    }\n",[1072,1139,1141],{"class":1074,"line":1140},7,[1072,1142,1143],{"class":1086},"};\n",[918,1145,1146],{},"这里出现的语法就是重载类型转换运算符的语法了：",[1148,1149,1150],"sdsc",{},[918,1151,1152],{},"\"operator\" 类型名 \"()\" 函数体",[918,1154,1155,1156,1159,1160,1163,1164,1167,1168,1172,1173,1175,1176,1178],{},"这里，",[948,1157,1158],{},"@\"operator\" 类型名@"," 的写法和之前的什么 ",[948,1161,1162],{},"operator+="," 啊 ",[948,1165,1166],{},"operator[]"," 啊是很像的；而且它不需要参数——因为类型转换是一元运算符，不再需要额外的右操作数了。但不一样的地方是它",[1169,1170,1171],"strong",{},"没有返回值类型","。这是因为，转换到 ",[948,1174,973],{}," 类型的运算符重载必然返回 ",[948,1177,973],{}," 类型，所以不用再次强调。",[918,1180,1181,1182,1185,1186,1055,1188,1190,1191,1193,1194,1197,1198,1201,1202,1205],{},"我们还是以 ",[948,1183,1184],{},"String"," 类举例。这一次，我们定义从 ",[948,1187,1184],{},[948,1189,1045],{}," 的转换：如果 ",[948,1192,1184],{}," 为空字符串 ",[948,1195,1196],{},"\"\""," 则转换到 ",[948,1199,1200],{},"false","；否则，转换到 ",[948,1203,1204],{},"true","。也就是说效果类似：",[1063,1207,1209],{"className":1065,"code":1208,"language":1067,"meta":1068,"style":1068},"#include \u003Ciostream>\nint main() {\n    String a;\n    \u002F\u002F [...]\n    if (a) { \u002F\u002F 这里调用从 String 到 bool 的隐式转换\n        std::cout \u003C\u003C \"String a is not empty!\" \u003C\u003C std::endl;\n    } else {\n        std::cout \u003C\u003C \"String a is empty. Aborted.\" \u003C\u003C std::endl;\n    }\n    \u002F\u002F 当然你也可以显式转换\n    bool isEmpty;\n    isEmpty = !bool(a);\n}\n",[948,1210,1211,1226,1239,1247,1252,1271,1308,1318,1346,1351,1357,1368,1390],{"__ignoreMap":1068},[1072,1212,1213,1216,1219,1223],{"class":1074,"line":1075},[1072,1214,1215],{"class":1124},"#include",[1072,1217,1218],{"class":1086}," \u003C",[1072,1220,1222],{"class":1221},"sfazB","iostream",[1072,1224,1225],{"class":1086},">\n",[1072,1227,1228,1231,1235,1237],{"class":1074,"line":1090},[1072,1229,1230],{"class":1078},"int",[1072,1232,1234],{"class":1233},"s2Zo4"," main",[1072,1236,1109],{"class":1086},[1072,1238,1087],{"class":1086},[1072,1240,1241,1245],{"class":1074,"line":1099},[1072,1242,1244],{"class":1243},"sTEyZ","    String a",[1072,1246,1131],{"class":1086},[1072,1248,1249],{"class":1074,"line":1114},[1072,1250,1251],{"class":1117},"    \u002F\u002F [...]\n",[1072,1253,1254,1257,1260,1262,1265,1268],{"class":1074,"line":1121},[1072,1255,1256],{"class":1124},"    if",[1072,1258,1259],{"class":1086}," (",[1072,1261,922],{"class":1243},[1072,1263,1264],{"class":1086},")",[1072,1266,1267],{"class":1086}," {",[1072,1269,1270],{"class":1117}," \u002F\u002F 这里调用从 String 到 bool 的隐式转换\n",[1072,1272,1273,1276,1279,1283,1286,1289,1292,1295,1298,1301,1303,1306],{"class":1074,"line":1134},[1072,1274,1275],{"class":1082},"        std",[1072,1277,1278],{"class":1086},"::",[1072,1280,1282],{"class":1281},"swJcz","cout ",[1072,1284,1285],{"class":1086},"\u003C\u003C",[1072,1287,1288],{"class":1086}," \"",[1072,1290,1291],{"class":1221},"String a is not empty!",[1072,1293,1294],{"class":1086},"\"",[1072,1296,1297],{"class":1086}," \u003C\u003C",[1072,1299,1300],{"class":1082}," std",[1072,1302,1278],{"class":1086},[1072,1304,1305],{"class":1281},"endl",[1072,1307,1131],{"class":1086},[1072,1309,1310,1313,1316],{"class":1074,"line":1140},[1072,1311,1312],{"class":1086},"    }",[1072,1314,1315],{"class":1124}," else",[1072,1317,1087],{"class":1086},[1072,1319,1321,1323,1325,1327,1329,1331,1334,1336,1338,1340,1342,1344],{"class":1074,"line":1320},8,[1072,1322,1275],{"class":1082},[1072,1324,1278],{"class":1086},[1072,1326,1282],{"class":1281},[1072,1328,1285],{"class":1086},[1072,1330,1288],{"class":1086},[1072,1332,1333],{"class":1221},"String a is empty. Aborted.",[1072,1335,1294],{"class":1086},[1072,1337,1297],{"class":1086},[1072,1339,1300],{"class":1082},[1072,1341,1278],{"class":1086},[1072,1343,1305],{"class":1281},[1072,1345,1131],{"class":1086},[1072,1347,1349],{"class":1074,"line":1348},9,[1072,1350,1137],{"class":1086},[1072,1352,1354],{"class":1074,"line":1353},10,[1072,1355,1356],{"class":1117},"    \u002F\u002F 当然你也可以显式转换\n",[1072,1358,1360,1363,1366],{"class":1074,"line":1359},11,[1072,1361,1362],{"class":1078},"    bool",[1072,1364,1365],{"class":1243}," isEmpty",[1072,1367,1131],{"class":1086},[1072,1369,1371,1374,1377,1380,1382,1385,1387],{"class":1074,"line":1370},12,[1072,1372,1373],{"class":1243},"    isEmpty ",[1072,1375,1376],{"class":1086},"=",[1072,1378,1379],{"class":1086}," !",[1072,1381,1045],{"class":1078},[1072,1383,1384],{"class":1086},"(",[1072,1386,922],{"class":1243},[1072,1388,1389],{"class":1086},");\n",[1072,1391,1393],{"class":1074,"line":1392},13,[1072,1394,1395],{"class":1086},"}\n",[918,1397,1398],{},"那么实现过程只要套用上面的语法就可以了：",[1063,1400,1402],{"className":1065,"code":1401,"language":1067,"meta":1068,"style":1068},"class String {\npublic:\n    \u002F\u002F [...]\n    operator bool() {\n        if (len == 0) return false;\n        else return true;\n    }\n};\n",[948,1403,1404,1413,1419,1423,1434,1458,1468,1472],{"__ignoreMap":1068},[1072,1405,1406,1408,1411],{"class":1074,"line":1075},[1072,1407,1079],{"class":1078},[1072,1409,1410],{"class":1082}," String",[1072,1412,1087],{"class":1086},[1072,1414,1415,1417],{"class":1074,"line":1090},[1072,1416,1093],{"class":1078},[1072,1418,1096],{"class":1086},[1072,1420,1421],{"class":1074,"line":1099},[1072,1422,1251],{"class":1117},[1072,1424,1425,1427,1430,1432],{"class":1074,"line":1114},[1072,1426,1103],{"class":1102},[1072,1428,1429],{"class":1082}," bool",[1072,1431,1109],{"class":1086},[1072,1433,1087],{"class":1086},[1072,1435,1436,1439,1441,1444,1447,1450,1452,1455],{"class":1074,"line":1121},[1072,1437,1438],{"class":1124},"        if",[1072,1440,1259],{"class":1086},[1072,1442,1443],{"class":1281},"len ",[1072,1445,1446],{"class":1086},"==",[1072,1448,1449],{"class":1102}," 0",[1072,1451,1264],{"class":1086},[1072,1453,1454],{"class":1124}," return",[1072,1456,1457],{"class":1086}," false;\n",[1072,1459,1460,1463,1465],{"class":1074,"line":1134},[1072,1461,1462],{"class":1124},"        else",[1072,1464,1454],{"class":1124},[1072,1466,1467],{"class":1086}," true;\n",[1072,1469,1470],{"class":1074,"line":1140},[1072,1471,1137],{"class":1086},[1072,1473,1474],{"class":1074,"line":1320},[1072,1475,1143],{"class":1086},[928,1477,1478],{"id":1478},"从别的类型转换到类类型",[918,1480,1481,1482,1484],{},"接下来我们尝试反过来：如果转换到的类型是类类型，但转换前的类型无法控制（比如是一个 ",[948,1483,1230],{}," 之类的），那么该怎么做呢？",[918,1486,1487,1488,1490,1491,1493,1494,1497,1498,1501,1502,1505],{},"如果用 ",[948,1489,1184],{}," 类举例的话，我想定义从一个 C 风格字符串到 ",[948,1492,1184],{}," 的转换。C 风格字符串是 ",[948,1495,1496],{},"const char[N]"," 类型的，而 ",[948,1499,1500],{},"N"," 是未知的，所以为了简便起见就用 ",[948,1503,1504],{},"const char*"," 类型好了。那么我们想要做的效果就是：",[1063,1507,1509],{"className":1065,"code":1508,"language":1067,"meta":1068,"style":1068},"#include \u003Ciostream>\n\u002F\u002F 截止目前的 String 类定义：https:\u002F\u002Fpaste.ubuntu.com\u002Fp\u002Fd4HYm4cZ4h\u002F\nint main() {\n    char a[]{\"Hello\"};\n    String b;\n    b = a; \u002F\u002F 赋值运算符右侧期待 String，但传入 const char* 发生转换\n    std::cout \u003C\u003C b.str \u003C\u003C std::endl; \u002F\u002F 应输出 \"Hello\"\n}\n",[948,1510,1511,1521,1526,1536,1556,1563,1579,1612],{"__ignoreMap":1068},[1072,1512,1513,1515,1517,1519],{"class":1074,"line":1075},[1072,1514,1215],{"class":1124},[1072,1516,1218],{"class":1086},[1072,1518,1222],{"class":1221},[1072,1520,1225],{"class":1086},[1072,1522,1523],{"class":1074,"line":1090},[1072,1524,1525],{"class":1117},"\u002F\u002F 截止目前的 String 类定义：https:\u002F\u002Fpaste.ubuntu.com\u002Fp\u002Fd4HYm4cZ4h\u002F\n",[1072,1527,1528,1530,1532,1534],{"class":1074,"line":1099},[1072,1529,1230],{"class":1078},[1072,1531,1234],{"class":1233},[1072,1533,1109],{"class":1086},[1072,1535,1087],{"class":1086},[1072,1537,1538,1541,1544,1547,1549,1552,1554],{"class":1074,"line":1114},[1072,1539,1540],{"class":1078},"    char",[1072,1542,1543],{"class":1243}," a[]",[1072,1545,1546],{"class":1086},"{",[1072,1548,1294],{"class":1086},[1072,1550,1551],{"class":1221},"Hello",[1072,1553,1294],{"class":1086},[1072,1555,1143],{"class":1086},[1072,1557,1558,1561],{"class":1074,"line":1121},[1072,1559,1560],{"class":1243},"    String b",[1072,1562,1131],{"class":1086},[1072,1564,1565,1568,1570,1573,1576],{"class":1074,"line":1134},[1072,1566,1567],{"class":1243},"    b ",[1072,1569,1376],{"class":1086},[1072,1571,1572],{"class":1243}," a",[1072,1574,1575],{"class":1086},";",[1072,1577,1578],{"class":1117}," \u002F\u002F 赋值运算符右侧期待 String，但传入 const char* 发生转换\n",[1072,1580,1581,1584,1586,1588,1590,1593,1596,1599,1601,1603,1605,1607,1609],{"class":1074,"line":1140},[1072,1582,1583],{"class":1082},"    std",[1072,1585,1278],{"class":1086},[1072,1587,1282],{"class":1243},[1072,1589,1285],{"class":1086},[1072,1591,1592],{"class":1243}," b",[1072,1594,1595],{"class":1086},".",[1072,1597,1598],{"class":1243},"str ",[1072,1600,1285],{"class":1086},[1072,1602,1300],{"class":1082},[1072,1604,1278],{"class":1086},[1072,1606,1305],{"class":1243},[1072,1608,1575],{"class":1086},[1072,1610,1611],{"class":1117}," \u002F\u002F 应输出 \"Hello\"\n",[1072,1613,1614],{"class":1074,"line":1320},[1072,1615,1395],{"class":1086},[918,1617,1618],{},"诶诶诶，你会神奇的发现：我们什么都没做怎么就编译通过了？而且运行的结果还是正确的！",[918,1620,1621],{},"这是因为，我们定义了这个东西：",[1063,1623,1625],{"className":1065,"code":1624,"language":1067,"meta":1068,"style":1068},"class String {\npublic:\n    String(const char* initVal);\n};\n",[948,1626,1627,1635,1641,1660],{"__ignoreMap":1068},[1072,1628,1629,1631,1633],{"class":1074,"line":1075},[1072,1630,1079],{"class":1078},[1072,1632,1410],{"class":1082},[1072,1634,1087],{"class":1086},[1072,1636,1637,1639],{"class":1074,"line":1090},[1072,1638,1093],{"class":1078},[1072,1640,1096],{"class":1086},[1072,1642,1643,1646,1648,1651,1654,1658],{"class":1074,"line":1099},[1072,1644,1645],{"class":1233},"    String",[1072,1647,1384],{"class":1086},[1072,1649,1650],{"class":1078},"const",[1072,1652,1653],{"class":1078}," char*",[1072,1655,1657],{"class":1656},"sHdIc"," initVal",[1072,1659,1389],{"class":1086},[1072,1661,1662],{"class":1074,"line":1114},[1072,1663,1143],{"class":1086},[918,1665,1666,1667,1669,1670,1055,1672,1674],{},"这是参数列表只有一个 ",[948,1668,1504],{}," 的构造函数。而这个构造函数实际上同样定义了从 ",[948,1671,1504],{},[948,1673,1184],{}," 的转换；而转换的过程恰恰就是调用构造函数的过程。所以说，有了这个构造函数我们可以做隐式类型转换：",[1063,1676,1678],{"className":1065,"code":1677,"language":1067,"meta":1068,"style":1068},"int main() {\n    String a;\n    a = \"abc\"; \u002F\u002F 从 const char* 隐式转换到 String，然后调用赋值运算符重载\n}\n",[948,1679,1680,1690,1696,1715],{"__ignoreMap":1068},[1072,1681,1682,1684,1686,1688],{"class":1074,"line":1075},[1072,1683,1230],{"class":1078},[1072,1685,1234],{"class":1233},[1072,1687,1109],{"class":1086},[1072,1689,1087],{"class":1086},[1072,1691,1692,1694],{"class":1074,"line":1090},[1072,1693,1244],{"class":1243},[1072,1695,1131],{"class":1086},[1072,1697,1698,1701,1703,1705,1708,1710,1712],{"class":1074,"line":1099},[1072,1699,1700],{"class":1243},"    a ",[1072,1702,1376],{"class":1086},[1072,1704,1288],{"class":1086},[1072,1706,1707],{"class":1221},"abc",[1072,1709,1294],{"class":1086},[1072,1711,1575],{"class":1086},[1072,1713,1714],{"class":1117}," \u002F\u002F 从 const char* 隐式转换到 String，然后调用赋值运算符重载\n",[1072,1716,1717],{"class":1074,"line":1114},[1072,1718,1395],{"class":1086},[918,1720,1721],{},"也可以做显式类型转换：",[1063,1723,1725],{"className":1065,"code":1724,"language":1067,"meta":1068,"style":1068},"int main() {\n    String a;\n    a = String(\"abc\"); \u002F\u002F String(\"abc\") 是类型转换表达式\n}\n",[948,1726,1727,1737,1743,1765],{"__ignoreMap":1068},[1072,1728,1729,1731,1733,1735],{"class":1074,"line":1075},[1072,1730,1230],{"class":1078},[1072,1732,1234],{"class":1233},[1072,1734,1109],{"class":1086},[1072,1736,1087],{"class":1086},[1072,1738,1739,1741],{"class":1074,"line":1090},[1072,1740,1244],{"class":1243},[1072,1742,1131],{"class":1086},[1072,1744,1745,1747,1749,1751,1753,1755,1757,1759,1762],{"class":1074,"line":1099},[1072,1746,1700],{"class":1243},[1072,1748,1376],{"class":1086},[1072,1750,1410],{"class":1233},[1072,1752,1384],{"class":1086},[1072,1754,1294],{"class":1086},[1072,1756,1707],{"class":1221},[1072,1758,1294],{"class":1086},[1072,1760,1761],{"class":1086},");",[1072,1763,1764],{"class":1117}," \u002F\u002F String(\"abc\") 是类型转换表达式\n",[1072,1766,1767],{"class":1074,"line":1114},[1072,1768,1395],{"class":1086},[918,1770,1771,1772,1774,1775,1777],{},"而且神奇地，这种类型转换表达式的写法和构造函数的定义神似。也有人管这种类型转换表达式叫做“构造临时量”。类似地，我们也可以将“零个值”转换为 ",[948,1773,1184],{},"（通过调用默认构造函数）或者将多于一个的值转换为 ",[948,1776,1184],{},"：",[1063,1779,1781],{"className":1065,"code":1780,"language":1067,"meta":1068,"style":1068},"int main() {\n    String a(\"Hello\");\n    a = String();       \u002F\u002F 现在 a 是 \"\"\n    a = String(5, '@'); \u002F\u002F 现在 a 是 \"@@@@@\"\n}\n",[948,1782,1783,1793,1810,1824,1854],{"__ignoreMap":1068},[1072,1784,1785,1787,1789,1791],{"class":1074,"line":1075},[1072,1786,1230],{"class":1078},[1072,1788,1234],{"class":1233},[1072,1790,1109],{"class":1086},[1072,1792,1087],{"class":1086},[1072,1794,1795,1798,1800,1802,1804,1806,1808],{"class":1074,"line":1090},[1072,1796,1797],{"class":1243},"    String ",[1072,1799,922],{"class":1233},[1072,1801,1384],{"class":1086},[1072,1803,1294],{"class":1086},[1072,1805,1551],{"class":1221},[1072,1807,1294],{"class":1086},[1072,1809,1389],{"class":1086},[1072,1811,1812,1814,1816,1818,1821],{"class":1074,"line":1099},[1072,1813,1700],{"class":1243},[1072,1815,1376],{"class":1086},[1072,1817,1410],{"class":1233},[1072,1819,1820],{"class":1086},"();",[1072,1822,1823],{"class":1117},"       \u002F\u002F 现在 a 是 \"\"\n",[1072,1825,1826,1828,1830,1832,1834,1837,1840,1843,1846,1849,1851],{"class":1074,"line":1114},[1072,1827,1700],{"class":1243},[1072,1829,1376],{"class":1086},[1072,1831,1410],{"class":1233},[1072,1833,1384],{"class":1086},[1072,1835,1836],{"class":1102},"5",[1072,1838,1839],{"class":1086},",",[1072,1841,1842],{"class":1086}," '",[1072,1844,1845],{"class":1221},"@",[1072,1847,1848],{"class":1086},"'",[1072,1850,1761],{"class":1086},[1072,1852,1853],{"class":1117}," \u002F\u002F 现在 a 是 \"@@@@@\"\n",[1072,1855,1856],{"class":1074,"line":1121},[1072,1857,1395],{"class":1086},[918,1859,1860,1861,1864,1865,1868],{},"目前而言，我们学过的所有构造函数都可以用于隐式或显式的类型转换。可以用于隐式类型转换的构造函数又被称为",[1169,1862,1863],{},"转换构造函数","（即非 ",[948,1866,1867],{},"explicit"," 说明的构造函数，见下文）。",[1870,1871,1872],"blockquote",{},[918,1873,1874,1875,1878,1879,1882,1883,1885],{},"类似地，也有构造聚合初始化临时值的列表风格转型运算符 ",[948,1876,1877],{},"@类型\"{\"值列表\"}\"@","。它会用 ",[948,1880,1881],{},"@值列表@"," 的值来大括号（聚合）初始化 ",[948,1884,962],{}," 变量，将初始化得到的对象作为表达式的结果。",[928,1887,1889,1891],{"id":1888},"explicit-关键字",[948,1890,1867],{}," 关键字",[918,1893,1894,1895,1897,1898,1055,1900,1902,1903,1906,1907,1909,1910,1912,1913,1915],{},"最后我们引入 ",[948,1896,1867],{}," 关键字。我们注意到隐式转换发生的过于频繁（而且可以有很多意想不到的隐式转换）。比如刚刚只定义了从 ",[948,1899,1504],{},[948,1901,1184],{}," 的转换，而刚刚 ",[948,1904,1905],{},"a = \"abc\""," 这个表达式其实是先从 ",[948,1908,1496],{}," 转换到 ",[948,1911,1504],{},"，然后再转换到 ",[948,1914,1184],{}," 的。这表明，在 C++ 里面，同一个地方可以允许非常多次的转换。",[918,1917,1918,1919,1055,1921,1923,1924,1926,1927,1929],{},"另一方面，我们又定义了从 ",[948,1920,1184],{},[948,1922,1045],{}," 的转换。但 ",[948,1925,1045],{}," 能转换到 ",[948,1928,1230],{},"，这就导致：",[1063,1931,1933],{"className":1065,"code":1932,"language":1067,"meta":1068,"style":1068},"int main() {\n    String a(\"42\");\n    int val{a}; \u002F\u002F 猜猜 val 是多少？\n}\n",[948,1934,1935,1945,1962,1980],{"__ignoreMap":1068},[1072,1936,1937,1939,1941,1943],{"class":1074,"line":1075},[1072,1938,1230],{"class":1078},[1072,1940,1234],{"class":1233},[1072,1942,1109],{"class":1086},[1072,1944,1087],{"class":1086},[1072,1946,1947,1949,1951,1953,1955,1958,1960],{"class":1074,"line":1090},[1072,1948,1797],{"class":1243},[1072,1950,922],{"class":1233},[1072,1952,1384],{"class":1086},[1072,1954,1294],{"class":1086},[1072,1956,1957],{"class":1221},"42",[1072,1959,1294],{"class":1086},[1072,1961,1389],{"class":1086},[1072,1963,1964,1967,1970,1972,1974,1977],{"class":1074,"line":1099},[1072,1965,1966],{"class":1078},"    int",[1072,1968,1969],{"class":1243}," val",[1072,1971,1546],{"class":1086},[1072,1973,922],{"class":1281},[1072,1975,1976],{"class":1086},"};",[1072,1978,1979],{"class":1117}," \u002F\u002F 猜猜 val 是多少？\n",[1072,1981,1982],{"class":1074,"line":1114},[1072,1983,1395],{"class":1086},[918,1985,1986,1987,1990,1991,1046,1993,1995,1996,1998,1999,2001,2002,1909,2005,2007,2008,2010,2011,1909,2013,2007,2015,2018,2019,2021],{},"显然 ",[948,1988,1989],{},"val"," 不能是 ",[948,1992,1957],{},[948,1994,1989],{}," 的值是从 ",[948,1997,1045],{}," 类型转换过来的，而 ",[948,2000,1184],{}," 值 ",[948,2003,2004],{},"\"42\"",[948,2006,1045],{}," 是 ",[948,2009,1204],{},"，",[948,2012,1204],{},[948,2014,1230],{},[948,2016,2017],{},"1","，所以答案是 ",[948,2020,2017],{},"。这个神奇的过程在某种程度上会加大编程者的心智负担，所以我们并不希望如此自由的转换。",[918,2023,2024,2025,2027,2028,2031],{},"于是 ",[948,2026,1867],{}," 关键字就呼之欲出了。它表明：这个转换",[1169,2029,2030],{},"只能用于显式转换","，不能用于隐式转换。",[1063,2033,2035],{"className":1065,"code":2034,"language":1067,"meta":1068,"style":1068},"struct B {};\nstruct C {};\nstruct A {\n    A() { }\n    explicit A(const C&) { }              \u002F\u002F 只允许显式的从 C 到 A 的转换\n    explicit operator B() { return B(); } \u002F\u002F 只允许显式的从 A 到 B 的转换\n};\nint main() {\n    A a; B b; C c;\n\u002F\u002F  a = c;    \u002F\u002F 不许隐式转换\n    a = A(c); \u002F\u002F 但可以显式转换\n\u002F\u002F  b = a;    \u002F\u002F 不许隐式转换\n    b = B(a); \u002F\u002F 但可以显式转换\n}\n",[948,2036,2037,2047,2056,2064,2076,2102,2126,2130,2140,2157,2165,2183,2190,2206],{"__ignoreMap":1068},[1072,2038,2039,2042,2044],{"class":1074,"line":1075},[1072,2040,2041],{"class":1078},"struct",[1072,2043,1106],{"class":1082},[1072,2045,2046],{"class":1086}," {};\n",[1072,2048,2049,2051,2054],{"class":1074,"line":1090},[1072,2050,2041],{"class":1078},[1072,2052,2053],{"class":1082}," C",[1072,2055,2046],{"class":1086},[1072,2057,2058,2060,2062],{"class":1074,"line":1099},[1072,2059,2041],{"class":1078},[1072,2061,1083],{"class":1082},[1072,2063,1087],{"class":1086},[1072,2065,2066,2069,2071,2073],{"class":1074,"line":1114},[1072,2067,2068],{"class":1233},"    A",[1072,2070,1109],{"class":1086},[1072,2072,1267],{"class":1086},[1072,2074,2075],{"class":1086}," }\n",[1072,2077,2078,2081,2083,2085,2087,2089,2092,2094,2096,2099],{"class":1074,"line":1121},[1072,2079,2080],{"class":1078},"    explicit",[1072,2082,1083],{"class":1233},[1072,2084,1384],{"class":1086},[1072,2086,1650],{"class":1078},[1072,2088,2053],{"class":1082},[1072,2090,2091],{"class":1078},"&",[1072,2093,1264],{"class":1086},[1072,2095,1267],{"class":1086},[1072,2097,2098],{"class":1086}," }",[1072,2100,2101],{"class":1117},"              \u002F\u002F 只允许显式的从 C 到 A 的转换\n",[1072,2103,2104,2106,2109,2111,2113,2115,2117,2119,2121,2123],{"class":1074,"line":1134},[1072,2105,2080],{"class":1078},[1072,2107,2108],{"class":1102}," operator",[1072,2110,1106],{"class":1082},[1072,2112,1109],{"class":1086},[1072,2114,1267],{"class":1086},[1072,2116,1454],{"class":1124},[1072,2118,1106],{"class":1233},[1072,2120,1820],{"class":1086},[1072,2122,2098],{"class":1086},[1072,2124,2125],{"class":1117}," \u002F\u002F 只允许显式的从 A 到 B 的转换\n",[1072,2127,2128],{"class":1074,"line":1140},[1072,2129,1143],{"class":1086},[1072,2131,2132,2134,2136,2138],{"class":1074,"line":1320},[1072,2133,1230],{"class":1078},[1072,2135,1234],{"class":1233},[1072,2137,1109],{"class":1086},[1072,2139,1087],{"class":1086},[1072,2141,2142,2145,2147,2150,2152,2155],{"class":1074,"line":1348},[1072,2143,2144],{"class":1243},"    A a",[1072,2146,1575],{"class":1086},[1072,2148,2149],{"class":1243}," B b",[1072,2151,1575],{"class":1086},[1072,2153,2154],{"class":1243}," C c",[1072,2156,1131],{"class":1086},[1072,2158,2159,2162],{"class":1074,"line":1353},[1072,2160,2161],{"class":1117},"\u002F\u002F  a = c;",[1072,2163,2164],{"class":1117},"    \u002F\u002F 不许隐式转换\n",[1072,2166,2167,2169,2171,2173,2175,2178,2180],{"class":1074,"line":1359},[1072,2168,1700],{"class":1243},[1072,2170,1376],{"class":1086},[1072,2172,1083],{"class":1233},[1072,2174,1384],{"class":1086},[1072,2176,2177],{"class":1243},"c",[1072,2179,1761],{"class":1086},[1072,2181,2182],{"class":1117}," \u002F\u002F 但可以显式转换\n",[1072,2184,2185,2188],{"class":1074,"line":1370},[1072,2186,2187],{"class":1117},"\u002F\u002F  b = a;",[1072,2189,2164],{"class":1117},[1072,2191,2192,2194,2196,2198,2200,2202,2204],{"class":1074,"line":1392},[1072,2193,1567],{"class":1243},[1072,2195,1376],{"class":1086},[1072,2197,1106],{"class":1233},[1072,2199,1384],{"class":1086},[1072,2201,922],{"class":1243},[1072,2203,1761],{"class":1086},[1072,2205,2182],{"class":1117},[1072,2207,2209],{"class":1074,"line":2208},14,[1072,2210,1395],{"class":1086},[918,2212,2213,2214,1055,2216,2218,2219,2221,2222,2224,2225,2227],{},"对于我们而言，我们不想要隐式的从 ",[948,2215,1184],{},[948,2217,1045],{}," 的转换，所以把它标记为 ",[948,2220,1867],{}," 的。但这是不是就引发了这样的问题：我们没有办法直接在 ",[948,2223,1030],{}," 里面用 ",[948,2226,1184],{}," 了？",[1063,2229,2231],{"className":1065,"code":2230,"language":1067,"meta":1068,"style":1068},"int main() {\n    String a;\n    if (a) { } \u002F\u002F a 不能隐式转换到 bool 了？\n}\n",[948,2232,2233,2243,2249,2266],{"__ignoreMap":1068},[1072,2234,2235,2237,2239,2241],{"class":1074,"line":1075},[1072,2236,1230],{"class":1078},[1072,2238,1234],{"class":1233},[1072,2240,1109],{"class":1086},[1072,2242,1087],{"class":1086},[1072,2244,2245,2247],{"class":1074,"line":1090},[1072,2246,1244],{"class":1243},[1072,2248,1131],{"class":1086},[1072,2250,2251,2253,2255,2257,2259,2261,2263],{"class":1074,"line":1099},[1072,2252,1256],{"class":1124},[1072,2254,1259],{"class":1086},[1072,2256,922],{"class":1243},[1072,2258,1264],{"class":1086},[1072,2260,1267],{"class":1086},[1072,2262,2098],{"class":1086},[1072,2264,2265],{"class":1117}," \u002F\u002F a 不能隐式转换到 bool 了？\n",[1072,2267,2268],{"class":1074,"line":1114},[1072,2269,1395],{"class":1086},[918,2271,2272],{},"实际上这个问题并不存在。因为 C++ 提供了一个例外判断，如果在形如：",[935,2274,2275,2284],{},[938,2276,2277,1031,2279,1031,2281,2283],{},[948,2278,1030],{},[948,2280,1038],{},[948,2282,1034],{}," 条件；",[938,2285,2286,1031,2289,1031,2292,2295,2296,2299],{},[948,2287,2288],{},"&&",[948,2290,2291],{},"||",[948,2293,2294],{},"!"," 操作数，",[948,2297,2298],{},"?:"," 第一操作数；",[918,2301,2302,2303,2306,2307,2309],{},"等这样的条件下，则",[1169,2304,2305],{},"也考虑","显式类型转换。这种例外判断称为“按语境转换到 ",[948,2308,1045],{},"”。",[2311,2312,2313],"style",{},"html pre.shiki code .spNyl, html code.shiki .spNyl{--shiki-light:#9C3EDA;--shiki-default:#C792EA;--shiki-dark:#C792EA}html pre.shiki code .sBMFI, html code.shiki .sBMFI{--shiki-light:#E2931D;--shiki-default:#FFCB6B;--shiki-dark:#FFCB6B}html pre.shiki code .sMK4o, html code.shiki .sMK4o{--shiki-light:#39ADB5;--shiki-default:#89DDFF;--shiki-dark:#89DDFF}html pre.shiki code .sbssI, html code.shiki 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