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hs 做一些操作\n",[939,1135,1137,1140,1143,1145,1147,1150,1153],{"class":941,"line":1136},7,[939,1138,1139],{"class":1038},"    if",[939,1141,1142],{"class":953}," (",[939,1144,995],{"class":1013},[939,1146,1017],{"class":953},[939,1148,1149],{"class":963},"hs",[939,1151,1152],{"class":953},"))",[939,1154,954],{"class":953},[939,1156,1158,1161,1164,1168,1171,1174,1177,1180,1183,1186,1188,1191],{"class":941,"line":1157},8,[939,1159,1160],{"class":949},"        std",[939,1162,1163],{"class":953},"::",[939,1165,1167],{"class":1166},"swJcz","cout ",[939,1169,1170],{"class":953},"\u003C\u003C",[939,1172,1173],{"class":953}," \"",[939,1175,1176],{"class":1074},"Ok",[939,1178,1179],{"class":953},"\"",[939,1181,1182],{"class":953}," \u003C\u003C",[939,1184,1185],{"class":949}," std",[939,1187,1163],{"class":953},[939,1189,1190],{"class":1166},"endl",[939,1192,1045],{"class":953},[939,1194,1196],{"class":941,"line":1195},9,[939,1197,1198],{"class":953},"    }\n",[939,1200,1202],{"class":941,"line":1201},10,[939,1203,1051],{"class":953},[926,1205,1206,1207,1211,1212,1214,1215,1217,1218,1220],{},"这一切看上去很不错。但是回想",[1208,1209,1210],"a",{"href":249},"函数执行","的过程，我们在调用 ",[922,1213,995],{}," 函数的过程中发生的参数传递是“复制”的——也就是说，这段程序需要将一个很大的数据从 ",[922,1216,1057],{}," 的内存复制到 ",[922,1219,995],{}," 的内存：",[1222,1223,1224],"sdsc",{},[926,1225,1226],{},"\"HugeStruct a{\"main 函数中的 hs\"};\"",[926,1228,1229,1230,1232,1233,1235,1236,1238],{},"然而，如此巨大开销的复制仅仅是为了做一些检查。那么我们为什么不直接检查 ",[922,1231,1057],{}," 函数中的 ",[922,1234,1149],{}," 呢？于是我们可以将 ",[922,1237,995],{}," 函数的参数类型改为引用：",[930,1240,1242],{"className":932,"code":1241,"language":934,"meta":935,"style":935},"bool check(HugeStruct& a) {\n    \u002F\u002F [...]\n    return \u002F* sth *\u002F;\n}\n",[922,1243,1244,1263,1268,1276],{"__ignoreMap":935},[939,1245,1246,1248,1250,1252,1254,1257,1259,1261],{"class":941,"line":942},[939,1247,1010],{"class":945},[939,1249,1014],{"class":1013},[939,1251,1017],{"class":953},[939,1253,999],{"class":949},[939,1255,1256],{"class":945},"&",[939,1258,1023],{"class":1022},[939,1260,1026],{"class":953},[939,1262,954],{"class":953},[939,1264,1265],{"class":941,"line":957},[939,1266,1267],{"class":982},"    \u002F\u002F [...]\n",[939,1269,1270,1272,1274],{"class":941,"line":986},[939,1271,1039],{"class":1038},[939,1273,1042],{"class":982},[939,1275,1045],{"class":953},[939,1277,1278],{"class":941,"line":1048},[939,1279,1051],{"class":953},[926,1281,1282],{},"此时，参数传递发生的是：",[1222,1284,1285],{},[926,1286,1287],{},"\"HugeStruct& a{\"main 函数中的 hs\"};\"",[926,1289,1290,1291,1293,1294,1296,1297,1299,1300,1304],{},"也就是说 ",[922,1292,1208],{}," 只是 ",[922,1295,1057],{}," 函数中 ",[922,1298,1149],{}," 的一个别名，而起别名这个操作是不需要复制任何数据的。所以，",[1301,1302,1303],"strong",{},"传递引用可以通过减少复制来提高性能","。",[926,1306,1307,1308,1310,1311,1313,1314,1316,1317,1319,1320,1322,1323,1326],{},"然而传递引用参数带来的另外一个问题是，如果 ",[922,1309,995],{}," 函数“不小心”对 ",[922,1312,1208],{}," 做了改动怎么办？在原先不传递引用的版本中，如果这些“不小心的改动”发生并不会影响到 ",[922,1315,1057],{}," 中 ",[922,1318,1149],{}," 的变化；但传递引用后这些变化就会立刻反映到 ",[922,1321,1057],{}," 中，导致意想不到的结果。所以，我们的解决方法是在引用声明中加上 ",[922,1324,1325],{},"const"," 限定防止其更改：",[930,1328,1330],{"className":932,"code":1329,"language":934,"meta":935,"style":935},"bool check(const HugeStruct& a) {\n    \u002F\u002F [...] 这里不可能对 a 进行修改\n    return \u002F* sth *\u002F;\n}\n",[922,1331,1332,1352,1357,1365],{"__ignoreMap":935},[939,1333,1334,1336,1338,1340,1342,1344,1346,1348,1350],{"class":941,"line":942},[939,1335,1010],{"class":945},[939,1337,1014],{"class":1013},[939,1339,1017],{"class":953},[939,1341,1325],{"class":945},[939,1343,950],{"class":949},[939,1345,1256],{"class":945},[939,1347,1023],{"class":1022},[939,1349,1026],{"class":953},[939,1351,954],{"class":953},[939,1353,1354],{"class":941,"line":957},[939,1355,1356],{"class":982},"    \u002F\u002F [...] 这里不可能对 a 进行修改\n",[939,1358,1359,1361,1363],{"class":941,"line":986},[939,1360,1039],{"class":1038},[939,1362,1042],{"class":982},[939,1364,1045],{"class":953},[939,1366,1367],{"class":941,"line":1048},[939,1368,1051],{"class":953},[926,1370,1371,1372,1374,1375,1377,1378,1382],{},"这就是从直观层面上，",[922,1373,924],{}," 这种形式声明的理解。不过实际上，",[922,1376,924],{}," 是作为一种“",[1208,1379,1381],{"href":1380},"\u002Fch04\u002Fpointer\u002Fpointer_usage#idx_%E5%B7%A6%E5%80%BC","左","右通吃”的引用而存在的。我们这里不做更多展开，如果感兴趣可以尝试理解下面的代码：",[930,1384,1386],{"className":932,"code":1385,"language":934,"meta":935,"style":935},"void passByVal(int a) {}\nvoid passByRef(int& a) {}\nvoid passByConstRef(const int& a) {}\nint main() {\n    int x{42};          \u002F\u002F x 是左值，42 是右值\n    passByVal(x);       \u002F\u002F 值传递左值，OK 但有复制开销\n    passByVal(42);      \u002F\u002F 值传递右值，OK 但有复制开销\n    passByRef(x);       \u002F\u002F 引用传递左值，OK\n\u002F\u002F  passByRef(42);      \u002F\u002F 引用传递右值，编译错误\n    passByConstRef(x);  \u002F\u002F “只读引用”传递左值，OK\n    passByConstRef(42); \u002F\u002F “只读引用”传递右值，OK\n}\n",[922,1387,1388,1407,1425,1445,1455,1475,1490,1503,1517,1525,1539,1553],{"__ignoreMap":935},[939,1389,1390,1393,1396,1398,1400,1402,1404],{"class":941,"line":942},[939,1391,1392],{"class":945},"void",[939,1394,1395],{"class":1013}," passByVal",[939,1397,1017],{"class":953},[939,1399,1111],{"class":945},[939,1401,1023],{"class":1022},[939,1403,1026],{"class":953},[939,1405,1406],{"class":953}," {}\n",[939,1408,1409,1411,1414,1416,1419,1421,1423],{"class":941,"line":957},[939,1410,1392],{"class":945},[939,1412,1413],{"class":1013}," passByRef",[939,1415,1017],{"class":953},[939,1417,1418],{"class":945},"int&",[939,1420,1023],{"class":1022},[939,1422,1026],{"class":953},[939,1424,1406],{"class":953},[939,1426,1427,1429,1432,1434,1436,1439,1441,1443],{"class":941,"line":986},[939,1428,1392],{"class":945},[939,1430,1431],{"class":1013}," passByConstRef",[939,1433,1017],{"class":953},[939,1435,1325],{"class":945},[939,1437,1438],{"class":945}," int&",[939,1440,1023],{"class":1022},[939,1442,1026],{"class":953},[939,1444,1406],{"class":953},[939,1446,1447,1449,1451,1453],{"class":941,"line":1048},[939,1448,1111],{"class":945},[939,1450,1114],{"class":1013},[939,1452,1117],{"class":953},[939,1454,954],{"class":953},[939,1456,1457,1460,1463,1466,1469,1472],{"class":941,"line":1122},[939,1458,1459],{"class":945},"    int",[939,1461,1462],{"class":963}," x",[939,1464,1465],{"class":953},"{",[939,1467,1468],{"class":970},"42",[939,1470,1471],{"class":953},"};",[939,1473,1474],{"class":982},"          \u002F\u002F x 是左值，42 是右值\n",[939,1476,1477,1480,1482,1485,1487],{"class":941,"line":1130},[939,1478,1479],{"class":1013},"    passByVal",[939,1481,1017],{"class":953},[939,1483,1484],{"class":963},"x",[939,1486,1103],{"class":953},[939,1488,1489],{"class":982},"       \u002F\u002F 值传递左值，OK 但有复制开销\n",[939,1491,1492,1494,1496,1498,1500],{"class":941,"line":1136},[939,1493,1479],{"class":1013},[939,1495,1017],{"class":953},[939,1497,1468],{"class":970},[939,1499,1103],{"class":953},[939,1501,1502],{"class":982},"      \u002F\u002F 值传递右值，OK 但有复制开销\n",[939,1504,1505,1508,1510,1512,1514],{"class":941,"line":1157},[939,1506,1507],{"class":1013},"    passByRef",[939,1509,1017],{"class":953},[939,1511,1484],{"class":963},[939,1513,1103],{"class":953},[939,1515,1516],{"class":982},"       \u002F\u002F 引用传递左值，OK\n",[939,1518,1519,1522],{"class":941,"line":1195},[939,1520,1521],{"class":982},"\u002F\u002F  passByRef(42);",[939,1523,1524],{"class":982},"      \u002F\u002F 引用传递右值，编译错误\n",[939,1526,1527,1530,1532,1534,1536],{"class":941,"line":1201},[939,1528,1529],{"class":1013},"    passByConstRef",[939,1531,1017],{"class":953},[939,1533,1484],{"class":963},[939,1535,1103],{"class":953},[939,1537,1538],{"class":982},"  \u002F\u002F “只读引用”传递左值，OK\n",[939,1540,1542,1544,1546,1548,1550],{"class":941,"line":1541},11,[939,1543,1529],{"class":1013},[939,1545,1017],{"class":953},[939,1547,1468],{"class":970},[939,1549,1103],{"class":953},[939,1551,1552],{"class":982}," \u002F\u002F “只读引用”传递右值，OK\n",[939,1554,1556],{"class":941,"line":1555},12,[939,1557,1051],{"class":953},[918,1559,427],{"id":1560},"复制构造函数-1",[926,1562,1563],{},"有一个非常重要的构造函数称为复制构造函数，长成这样：",[930,1565,1567],{"className":932,"code":1566,"language":934,"meta":935,"style":935},"T(const T&);\n",[922,1568,1569],{"__ignoreMap":935},[939,1570,1571,1574,1576,1578,1581],{"class":941,"line":942},[939,1572,1573],{"class":1013},"T",[939,1575,1017],{"class":953},[939,1577,1325],{"class":945},[939,1579,1580],{"class":963}," T",[939,1582,1583],{"class":953},"&);\n",[926,1585,1586,1587,1589,1590,1592],{},"也就是接受 ",[922,1588,924],{}," 类型的参数，其中 ",[922,1591,1573],{}," 恰好是自己。它的特殊之处是，在需要做“复制”的操作时会调用这个构造函数。这样说起来很抽象，请看下面的例子：",[930,1594,1596],{"className":932,"code":1595,"language":934,"meta":935,"style":935},"#include \u003Ciostream>\nstruct S {\n    int data;\n    S(int d) {      \u002F\u002F 构造函数重载 #1\n        data = d;\n    }\n    S(const S& s) { \u002F\u002F 构造函数重载 #2，复制构造\n        std::cout \u003C\u003C \"copy constructor called\" \u003C\u003C std::endl;\n    }\n};\nvoid f(S a) { }\nint main() {\n    S sth(42); \u002F\u002F 调用重载 #1\n    f(sth);\n}\n",[922,1597,1598,1608,1617,1625,1645,1657,1661,1683,1710,1714,1718,1739,1749,1767,1780],{"__ignoreMap":935},[939,1599,1600,1602,1604,1606],{"class":941,"line":942},[939,1601,1068],{"class":1038},[939,1603,1071],{"class":953},[939,1605,1075],{"class":1074},[939,1607,1078],{"class":953},[939,1609,1610,1612,1615],{"class":941,"line":957},[939,1611,946],{"class":945},[939,1613,1614],{"class":949}," S",[939,1616,954],{"class":953},[939,1618,1619,1621,1623],{"class":941,"line":986},[939,1620,1459],{"class":945},[939,1622,964],{"class":1166},[939,1624,1045],{"class":953},[939,1626,1627,1630,1632,1634,1637,1639,1642],{"class":941,"line":1048},[939,1628,1629],{"class":1013},"    S",[939,1631,1017],{"class":953},[939,1633,1111],{"class":945},[939,1635,1636],{"class":1022}," d",[939,1638,1026],{"class":953},[939,1640,1641],{"class":953}," {",[939,1643,1644],{"class":982},"      \u002F\u002F 构造函数重载 #1\n",[939,1646,1647,1650,1653,1655],{"class":941,"line":1122},[939,1648,1649],{"class":1166},"        data ",[939,1651,1652],{"class":953},"=",[939,1654,1636],{"class":1166},[939,1656,1045],{"class":953},[939,1658,1659],{"class":941,"line":1130},[939,1660,1198],{"class":953},[939,1662,1663,1665,1667,1669,1671,1673,1676,1678,1680],{"class":941,"line":1136},[939,1664,1629],{"class":1013},[939,1666,1017],{"class":953},[939,1668,1325],{"class":945},[939,1670,1614],{"class":949},[939,1672,1256],{"class":945},[939,1674,1675],{"class":1022}," s",[939,1677,1026],{"class":953},[939,1679,1641],{"class":953},[939,1681,1682],{"class":982}," \u002F\u002F 构造函数重载 #2，复制构造\n",[939,1684,1685,1687,1689,1691,1693,1695,1698,1700,1702,1704,1706,1708],{"class":941,"line":1157},[939,1686,1160],{"class":949},[939,1688,1163],{"class":953},[939,1690,1167],{"class":1166},[939,1692,1170],{"class":953},[939,1694,1173],{"class":953},[939,1696,1697],{"class":1074},"copy constructor called",[939,1699,1179],{"class":953},[939,1701,1182],{"class":953},[939,1703,1185],{"class":949},[939,1705,1163],{"class":953},[939,1707,1190],{"class":1166},[939,1709,1045],{"class":953},[939,1711,1712],{"class":941,"line":1195},[939,1713,1198],{"class":953},[939,1715,1716],{"class":941,"line":1201},[939,1717,989],{"class":953},[939,1719,1720,1722,1725,1727,1730,1732,1734,1736],{"class":941,"line":1541},[939,1721,1392],{"class":945},[939,1723,1724],{"class":1013}," f",[939,1726,1017],{"class":953},[939,1728,1729],{"class":949},"S",[939,1731,1023],{"class":1022},[939,1733,1026],{"class":953},[939,1735,1641],{"class":953},[939,1737,1738],{"class":953}," }\n",[939,1740,1741,1743,1745,1747],{"class":941,"line":1555},[939,1742,1111],{"class":945},[939,1744,1114],{"class":1013},[939,1746,1117],{"class":953},[939,1748,954],{"class":953},[939,1750,1752,1755,1758,1760,1762,1764],{"class":941,"line":1751},13,[939,1753,1754],{"class":963},"    S ",[939,1756,1757],{"class":1013},"sth",[939,1759,1017],{"class":953},[939,1761,1468],{"class":970},[939,1763,1103],{"class":953},[939,1765,1766],{"class":982}," \u002F\u002F 调用重载 #1\n",[939,1768,1770,1773,1775,1777],{"class":941,"line":1769},14,[939,1771,1772],{"class":1013},"    f",[939,1774,1017],{"class":953},[939,1776,1757],{"class":963},[939,1778,1779],{"class":953},");\n",[939,1781,1783],{"class":941,"line":1782},15,[939,1784,1051],{"class":953},[926,1786,1787,1788,1790,1791,1794,1795,1797,1798,1801,1802,1805],{},"你可能会很意外地发现程序输出了 ",[922,1789,1697],{},"。我们明明没有手动调用过这个重载，为什么会输出东西呢？原因就在 ",[922,1792,1793],{},"f(sth);"," 这个函数调用上。因为这个函数是按值传递的，所以正如之前所说，它会经过一次复制的操作，把数据从 ",[922,1796,1057],{}," 的内存拷到 ",[922,1799,1800],{},"f"," 的内存。而这次复制就是通过调用重载 ",[922,1803,1804],{},"#2"," 来实现的。也就是说，参数传递如同：",[1222,1807,1808],{},[926,1809,1810],{},"\"S a(\"main函数中的 sth\");\"",[926,1812,1813,1814,1818],{},"类似地，在函数返回时也",[1815,1816,1817],"em",{},"可能","会发生值的复制。假设下面的场景：",[930,1820,1822],{"className":932,"code":1821,"language":934,"meta":935,"style":935},"S g() {\n    S sth(42); \u002F\u002F 调用重载 #1\n    return sth;\n}\nint main() {\n    S a(36); \u002F\u002F 调用重载 #1\n    a = g(); \u002F\u002F 返回值时调用重载 #2\n}\n",[922,1823,1824,1835,1849,1858,1862,1872,1887,1902],{"__ignoreMap":935},[939,1825,1826,1828,1831,1833],{"class":941,"line":942},[939,1827,1729],{"class":949},[939,1829,1830],{"class":1013}," g",[939,1832,1117],{"class":953},[939,1834,954],{"class":953},[939,1836,1837,1839,1841,1843,1845,1847],{"class":941,"line":957},[939,1838,1754],{"class":963},[939,1840,1757],{"class":1013},[939,1842,1017],{"class":953},[939,1844,1468],{"class":970},[939,1846,1103],{"class":953},[939,1848,1766],{"class":982},[939,1850,1851,1853,1856],{"class":941,"line":986},[939,1852,1039],{"class":1038},[939,1854,1855],{"class":963}," sth",[939,1857,1045],{"class":953},[939,1859,1860],{"class":941,"line":1048},[939,1861,1051],{"class":953},[939,1863,1864,1866,1868,1870],{"class":941,"line":1122},[939,1865,1111],{"class":945},[939,1867,1114],{"class":1013},[939,1869,1117],{"class":953},[939,1871,954],{"class":953},[939,1873,1874,1876,1878,1880,1883,1885],{"class":941,"line":1130},[939,1875,1754],{"class":963},[939,1877,1208],{"class":1013},[939,1879,1017],{"class":953},[939,1881,1882],{"class":970},"36",[939,1884,1103],{"class":953},[939,1886,1766],{"class":982},[939,1888,1889,1892,1894,1896,1899],{"class":941,"line":1136},[939,1890,1891],{"class":963},"    a ",[939,1893,1652],{"class":953},[939,1895,1830],{"class":1013},[939,1897,1898],{"class":953},"();",[939,1900,1901],{"class":982}," \u002F\u002F 返回值时调用重载 #2\n",[939,1903,1904],{"class":941,"line":1157},[939,1905,1051],{"class":953},[926,1907,1908,1909,1911,1912,1914,1915,1918,1919,1217,1922,1924,1925,1927],{},"那么这段函数",[1815,1910,1817],{},"同样会输出 ",[922,1913,1697],{},"。这是因为，",[922,1916,1917],{},"g()"," 这个表达式的结果需要从 ",[922,1920,1921],{},"g",[922,1923,1057],{}," 的内存，此时需要调用复制构造函数。但是，这种情形经常会被优化——编译器有时会直接在 ",[922,1926,1057],{}," 的内存完成所有的任务从而避免复制。这种优化称为“具名返回值优化”（Named Return Value Optimization, NRVO）。",[1929,1930,1931],"blockquote",{},[926,1932,1933,1934,1937,1938,1944],{},"对于 g++ 和 clang++ 编译器，启用 ",[922,1935,1936],{},"-fno-elide-constructors"," 开关来避免具名返回值优化。但非具名（匿名）的返回值优化是 C++17 所",[1208,1939,1943],{"href":1940,"rel":1941},"https:\u002F\u002Fzh.cppreference.com\u002Fw\u002Fcpp\u002Flanguage\u002Fcopy_elision",[1942],"nofollow","要求","的。",[1929,1946,1947],{},[926,1948,1949,1950,1953,1954,1953,1957,1960,1961,1963],{},"事实上，形如 ",[922,1951,1952],{},"T(T&);"," ",[922,1955,1956],{},"T(volatile T&);",[922,1958,1959],{},"T(const volatile T&);"," 的构造函数都是复制构造函数。但不带 ",[922,1962,1325],{}," 限定的复制构造参数是各种意义上违反直觉的，所以我们并不推荐。",[1965,1966,1967],"style",{},"html pre.shiki code .spNyl, html code.shiki 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.swJcz{--shiki-light:#E53935;--shiki-default:#F07178;--shiki-dark:#F07178}",{"title":935,"searchDepth":942,"depth":957,"links":1969},[1970,1971],{"id":920,"depth":957,"text":924},{"id":1560,"depth":957,"text":427},"md",null,{},{"icon":416},{"title":427,"description":427},"oqDCbXKAza2LL000rLSZw_FuxxPX5uPQoo-YsPzXk0Y",[1979,1981],{"title":423,"path":424,"stem":425,"description":1980,"icon":416,"children":-1},"正如上一节结尾所属，构造函数（Constructor function）是为了解决初始化的问题而存在的。",{"title":431,"path":432,"stem":433,"description":1982,"icon":416,"children":-1},"有些细心的读者可能发现，按值传递参数是非常常见的场景，也就是说调用复制构造函数应当是经常发生的事情。",1784618097096]