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f",[940,1160,1016],{"class":950},[940,1162,1163],{"class":1095},"c",[940,1165,1036],{"class":950},[940,1167,1169],{"class":942,"line":1168},17,[940,1170,1117],{"class":950},[918,1172,1173,1174,1176,1177,1180,1181,1183,1184,1186,1187,1189,1190,1192,1193,1195,1196,1198],{},"解释一下：类模板 ",[922,1175,924],{}," 中有一个私有成员 ",[922,1178,1179],{},"mem","，我想通过 ",[922,1182,928],{}," 这个函数来修改它。所以设置 ",[922,1185,928],{}," 为 ",[922,1188,924],{}," 的友元。第 6 行的代码做了这件事情。然后，main 函数尝试去用 ",[922,1191,928],{}," 来修改",[922,1194,924],{}," 类型的对象 ",[922,1197,1163],{},"。看上去很美好，但它非常离奇地给出了链接错误：",[931,1200,1205],{"className":1201,"code":1203,"language":1204},[1202],"language-text","ld: ...main.o:...main.cpp:16: undefined reference to `f(C\u003Cint>&)'\n","text",[922,1206,1203],{"__ignoreMap":936},[918,1208,1209,1210,1213],{},"它的意思是 ",[922,1211,1212],{},"void f(C\u003Cint>& c);"," 这个函数的定义没有找到。为什么呢？明明第 9 行到 12 行就是这个定义啊？不过我们细致地进一步分析一下，就知道问题的所在了。",[918,1215,1216,1217,1219,1220,1222,1223,1226,1227,1229,1230,1232],{},"首先，这里 ",[922,1218,924],{}," 和 ",[922,1221,928],{}," 都是模板。所以如果不实例化这些模板，那么就不存在对应的函数或类的定义。然后，main 函数中使用了 ",[922,1224,1225],{},"C\u003Cint>","，所以以实参 ",[922,1228,1128],{}," 实例化模板 ",[922,1231,924],{},"，得到这样的实例化结果：",[931,1234,1236],{"className":933,"code":1235,"language":935,"meta":936,"style":936},"class \u002F* C\u003Cint> *\u002F {\nprivate:\n    int mem;\npublic:\n    friend void f(C\u003Cint>& c);\n};\n",[922,1237,1238,1248,1254,1264,1270,1294],{"__ignoreMap":936},[940,1239,1240,1242,1246],{"class":942,"line":943},[940,1241,967],{"class":946},[940,1243,1245],{"class":1244},"sHwdD"," \u002F* C\u003Cint> *\u002F",[940,1247,973],{"class":950},[940,1249,1250,1252],{"class":942,"line":964},[940,1251,979],{"class":946},[940,1253,982],{"class":950},[940,1255,1256,1259,1262],{"class":942,"line":976},[940,1257,1258],{"class":946},"    int",[940,1260,1261],{"class":988}," mem",[940,1263,992],{"class":950},[940,1265,1266,1268],{"class":942,"line":985},[940,1267,998],{"class":946},[940,1269,982],{"class":950},[940,1271,1272,1274,1276,1278,1280,1282,1284,1286,1288,1290,1292],{"class":942,"line":995},[940,1273,1006],{"class":946},[940,1275,1009],{"class":946},[940,1277,1013],{"class":1012},[940,1279,1016],{"class":950},[940,1281,924],{"class":957},[940,1283,951],{"class":950},[940,1285,1128],{"class":946},[940,1287,1026],{"class":950},[940,1289,1029],{"class":946},[940,1291,1033],{"class":1032},[940,1293,1036],{"class":950},[940,1295,1296],{"class":942,"line":1003},[940,1297,1042],{"class":950},[918,1299,1300,1301,1303,1304,1306,1307,1310,1311,1313,1314,1316,1317,1319,1320,1322,1323,1327,1328,1330,1331,1333,1334,1336],{},"也就是说，",[922,1302,1225],{}," 期望一个友元函数 ",[922,1305,1212],{},"。然后，main 函数调用 ",[922,1308,1309],{},"f(c)","。注意，这里 ",[922,1312,1163],{}," 已经是 ",[922,1315,1225],{}," 类型的，而它拥有一个名字叫 ",[922,1318,928],{}," 的友元，所以编译器知道将来某个地方（可能是稍后，可能是其它的翻译单元）存在一个 ",[922,1321,1212],{}," 这样的函数的定义。",[1324,1325,1326],"strong",{},"当一个普通函数和函数模板同名时，编译器总是选择普通函数","。所以，这里并不会把 ",[922,1329,928],{}," 当成第 9 行的函数模板来调用，而是 ",[922,1332,1212],{}," 这个普通函数——毕竟 ",[922,1335,924],{}," 中友元声明了这样一个函数嘛。",[1338,1339,1340],"blockquote",{},[918,1341,1342],{},"像这样，将友元函数纳入函数调用考虑范围的做法称为实参依赖查找（Argument dependent Lookup, ADL）。",[918,1344,1345,1346,1348,1349,1355],{},"然而，直到最后链接阶段也没能发现 ",[922,1347,1212],{}," 这个函数的定义。最后，链接错误就出现了。你会发现，整个过程中，我们手写的",[1324,1350,1351,1352,1354],{},"函数模板 ",[922,1353,928],{}," 始终没有被实例化","。这就是问题的根源。",[918,1357,1358],{},"那么怎么办呢？关于这个问题，可以有许多种办法来解决。最简单的一种，是将友元函数定义放在类中。",[931,1360,1362],{"className":933,"code":1361,"language":935,"meta":936,"style":936},"template\u003Ctypename T>\nclass C {\nprivate:\n    T mem;\npublic:\n    friend void f(C\u003CT>& c) {\n        c.mem +=1;\n    }\n};\n\nint main() {\n    C\u003Cint> c;\n    f(c);\n}\n",[922,1363,1364,1376,1384,1390,1396,1402,1428,1445,1450,1454,1458,1468,1482,1492],{"__ignoreMap":936},[940,1365,1366,1368,1370,1372,1374],{"class":942,"line":943},[940,1367,947],{"class":946},[940,1369,951],{"class":950},[940,1371,954],{"class":946},[940,1373,958],{"class":957},[940,1375,961],{"class":950},[940,1377,1378,1380,1382],{"class":942,"line":964},[940,1379,967],{"class":946},[940,1381,970],{"class":957},[940,1383,973],{"class":950},[940,1385,1386,1388],{"class":942,"line":976},[940,1387,979],{"class":946},[940,1389,982],{"class":950},[940,1391,1392,1394],{"class":942,"line":985},[940,1393,989],{"class":988},[940,1395,992],{"class":950},[940,1397,1398,1400],{"class":942,"line":995},[940,1399,998],{"class":946},[940,1401,982],{"class":950},[940,1403,1404,1406,1408,1410,1412,1414,1416,1418,1420,1422,1424,1426],{"class":942,"line":1003},[940,1405,1006],{"class":946},[940,1407,1009],{"class":946},[940,1409,1013],{"class":1012},[940,1411,1016],{"class":950},[940,1413,924],{"class":957},[940,1415,951],{"class":950},[940,1417,1023],{"class":957},[940,1419,1026],{"class":950},[940,1421,1029],{"class":946},[940,1423,1033],{"class":1032},[940,1425,1087],{"class":950},[940,1427,973],{"class":950},[940,1429,1430,1433,1435,1437,1440,1443],{"class":942,"line":1039},[940,1431,1432],{"class":1095},"        c",[940,1434,1099],{"class":950},[940,1436,1179],{"class":1095},[940,1438,1439],{"class":950}," +=",[940,1441,1442],{"class":1108},"1",[940,1444,992],{"class":950},[940,1446,1447],{"class":942,"line":1045},[940,1448,1449],{"class":950},"    }\n",[940,1451,1452],{"class":942,"line":1052},[940,1453,1042],{"class":950},[940,1455,1456],{"class":942,"line":1065},[940,1457,1049],{"emptyLinePlaceholder":1048},[940,1459,1460,1462,1464,1466],{"class":942,"line":1092},[940,1461,1128],{"class":946},[940,1463,1131],{"class":1012},[940,1465,1134],{"class":950},[940,1467,973],{"class":950},[940,1469,1470,1472,1474,1476,1478,1480],{"class":942,"line":1114},[940,1471,1142],{"class":1095},[940,1473,951],{"class":950},[940,1475,1128],{"class":946},[940,1477,1026],{"class":950},[940,1479,1033],{"class":1095},[940,1481,992],{"class":950},[940,1483,1484,1486,1488,1490],{"class":942,"line":1120},[940,1485,1158],{"class":1012},[940,1487,1016],{"class":950},[940,1489,1163],{"class":1095},[940,1491,1036],{"class":950},[940,1493,1494],{"class":942,"line":1125},[940,1495,1117],{"class":950},[918,1497,1498,1499,1501],{},"这时，编译器在找到 ",[922,1500,1212],{}," 这个函数的声明也同时找到了它的定义。于是，链接通过。尽管我在第六章中没有强调这种写法；但在类模板的语境下，这反而是最佳的解决方案。",[918,1503,1504],{},"第二种办法是声明友元模板。（注意，友元模板和友元函数、友元类是并列的关系。友元模板不是“生成友元的模板”的意思。）友元模板使得一个模板生成的所有实例都成为其友元。具体的写法是这样的：",[931,1506,1508],{"className":933,"code":1507,"language":935,"meta":936,"style":936},"template\u003Ctypename T>\nclass C {\nprivate:\n    T mem;\npublic:\n    template\u003Ctypename U>\n    friend void f(C\u003CU>& c);\n};\n\ntemplate\u003Ctypename U>\nvoid f(C\u003CU>& c) {\n    c.mem += 1;\n}\n\nint main() {\n    C\u003Cint> c;\n    f(c);\n}\n",[922,1509,1510,1522,1530,1536,1542,1548,1562,1587,1591,1595,1607,1631,1645,1649,1653,1663,1677,1687],{"__ignoreMap":936},[940,1511,1512,1514,1516,1518,1520],{"class":942,"line":943},[940,1513,947],{"class":946},[940,1515,951],{"class":950},[940,1517,954],{"class":946},[940,1519,958],{"class":957},[940,1521,961],{"class":950},[940,1523,1524,1526,1528],{"class":942,"line":964},[940,1525,967],{"class":946},[940,1527,970],{"class":957},[940,1529,973],{"class":950},[940,1531,1532,1534],{"class":942,"line":976},[940,1533,979],{"class":946},[940,1535,982],{"class":950},[940,1537,1538,1540],{"class":942,"line":985},[940,1539,989],{"class":988},[940,1541,992],{"class":950},[940,1543,1544,1546],{"class":942,"line":995},[940,1545,998],{"class":946},[940,1547,982],{"class":950},[940,1549,1550,1553,1555,1557,1560],{"class":942,"line":1003},[940,1551,1552],{"class":946},"    template",[940,1554,951],{"class":950},[940,1556,954],{"class":946},[940,1558,1559],{"class":957}," U",[940,1561,961],{"class":950},[940,1563,1564,1566,1568,1570,1572,1574,1576,1579,1581,1583,1585],{"class":942,"line":1039},[940,1565,1006],{"class":946},[940,1567,1009],{"class":946},[940,1569,1013],{"class":1012},[940,1571,1016],{"class":950},[940,1573,924],{"class":957},[940,1575,951],{"class":950},[940,1577,1578],{"class":957},"U",[940,1580,1026],{"class":950},[940,1582,1029],{"class":946},[940,1584,1033],{"class":1032},[940,1586,1036],{"class":950},[940,1588,1589],{"class":942,"line":1045},[940,1590,1042],{"class":950},[940,1592,1593],{"class":942,"line":1052},[940,1594,1049],{"emptyLinePlaceholder":1048},[940,1596,1597,1599,1601,1603,1605],{"class":942,"line":1065},[940,1598,947],{"class":946},[940,1600,951],{"class":950},[940,1602,954],{"class":946},[940,1604,1559],{"class":957},[940,1606,961],{"class":950},[940,1608,1609,1611,1613,1615,1617,1619,1621,1623,1625,1627,1629],{"class":942,"line":1092},[940,1610,1068],{"class":946},[940,1612,1013],{"class":1012},[940,1614,1016],{"class":950},[940,1616,924],{"class":957},[940,1618,951],{"class":950},[940,1620,1578],{"class":957},[940,1622,1026],{"class":950},[940,1624,1029],{"class":946},[940,1626,1033],{"class":1032},[940,1628,1087],{"class":950},[940,1630,973],{"class":950},[940,1632,1633,1635,1637,1639,1641,1643],{"class":942,"line":1114},[940,1634,1096],{"class":1095},[940,1636,1099],{"class":950},[940,1638,1102],{"class":1095},[940,1640,1105],{"class":950},[940,1642,1109],{"class":1108},[940,1644,992],{"class":950},[940,1646,1647],{"class":942,"line":1120},[940,1648,1117],{"class":950},[940,1650,1651],{"class":942,"line":1125},[940,1652,1049],{"emptyLinePlaceholder":1048},[940,1654,1655,1657,1659,1661],{"class":942,"line":1139},[940,1656,1128],{"class":946},[940,1658,1131],{"class":1012},[940,1660,1134],{"class":950},[940,1662,973],{"class":950},[940,1664,1665,1667,1669,1671,1673,1675],{"class":942,"line":1155},[940,1666,1142],{"class":1095},[940,1668,951],{"class":950},[940,1670,1128],{"class":946},[940,1672,1026],{"class":950},[940,1674,1033],{"class":1095},[940,1676,992],{"class":950},[940,1678,1679,1681,1683,1685],{"class":942,"line":1168},[940,1680,1158],{"class":1012},[940,1682,1016],{"class":950},[940,1684,1163],{"class":1095},[940,1686,1036],{"class":950},[940,1688,1690],{"class":942,"line":1689},18,[940,1691,1117],{"class":950},[918,1693,1694,1697,1698,1701,1702,1705],{},[922,1695,1696],{},"template\u003Ctypename U> friend void f(C\u003CU>& c);"," 就是友元模板。这里，它的意思是：模板 ",[922,1699,1700],{},"template\u003Ctypename U> void f(C\u003CU>& c);"," 这个模板的每一个实例都是 ",[922,1703,1704],{},"C\u003CT>"," 的朋友。",[918,1707,1708,1709,1711,1712,1715],{},"当这样做时， ",[922,1710,1309],{}," 中为编译器所考虑的友元将是模板 ",[922,1713,1714],{},"template\u003Ctypename U> void f(C\u003CU>& c)"," 的一个实例化结果。为了实例化它，编译器找到了它在第 10 行的定义。于是，实例化得以进行，然后一切就顺利完成了。",[918,1717,1718,1719,1721,1722,1724],{},"最后一种常见的解决办法是声明一个 ",[922,1720,928],{}," 模板的 ",[922,1723,1023],{}," 特化作为其友元。这其实是最符合语义的一种写法，但因为我们没有讲什么叫模板的“特化”，所以这里只是科普一下不必深究。总的来说，最初的类内定义友元的方法仍然是最推荐的写法。",[931,1726,1728],{"className":933,"code":1727,"language":935,"meta":936,"style":936},"template\u003Ctypename T>\nclass C;\n\n\u002F\u002F 由于 C 中用到了 f 模板，所以必须提前声明 f\n\u002F\u002F 但 f 声明中又用到了 C，所以还需要前置声明 C\ntemplate\u003Ctypename T>\nvoid f(C\u003CT>& c);\n\ntemplate\u003Ctypename T>\nclass C {\nprivate:\n    T mem;\npublic:\n    \u002F\u002F 声明模板 f 中只有 f\u003CT> 是 C\u003CT> 的友元\n    \u002F\u002F 编译器会去为此去实例化 f\u003CT>\n    friend void f\u003CT>(C\u003CT>& c);\n};\n\ntemplate\u003Ctypename T>\nvoid f(C\u003CT>& c) {\n    c.mem += 1;\n}\n\nint main() {\n    C\u003Cint> c;\n    f(c);\n}\n",[922,1729,1730,1742,1750,1754,1759,1764,1776,1798,1802,1814,1822,1828,1834,1840,1845,1850,1878,1882,1886,1899,1924,1939,1944,1949,1960,1975,1986],{"__ignoreMap":936},[940,1731,1732,1734,1736,1738,1740],{"class":942,"line":943},[940,1733,947],{"class":946},[940,1735,951],{"class":950},[940,1737,954],{"class":946},[940,1739,958],{"class":957},[940,1741,961],{"class":950},[940,1743,1744,1746,1748],{"class":942,"line":964},[940,1745,967],{"class":946},[940,1747,970],{"class":957},[940,1749,992],{"class":950},[940,1751,1752],{"class":942,"line":976},[940,1753,1049],{"emptyLinePlaceholder":1048},[940,1755,1756],{"class":942,"line":985},[940,1757,1758],{"class":1244},"\u002F\u002F 由于 C 中用到了 f 模板，所以必须提前声明 f\n",[940,1760,1761],{"class":942,"line":995},[940,1762,1763],{"class":1244},"\u002F\u002F 但 f 声明中又用到了 C，所以还需要前置声明 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