string_view 和 basic_string<char> 之间的联系是什么？为什么 string_view 示例代码不起作用？

Question

我已经从 Bjarne Stroustrup 的C++ 之旅中复制了代码来测试字符串视图，但我不断收到错误：

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error: no matching function for call to \xe2\x80\x98std::__cxx11::basic_string<char>::basic_string(std::basic_string_view<char>::size_type\n

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我正在使用 VS CodeWSL 2和Ubuntu 20.04gcc-11

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在 中main.cpp，我有：

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#include <iostream>\n#include "strings.h"\n\nusing namespace std;\n\nint main () {\n    string king = "Harold";\n    auto s1 = cat(king, "William");\n}\n

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中strings.h，我有以下内容。该函数按照教科书上的内容复制。我把它打出来是为了避免*出现具有不同编码的特殊字符。

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#pragma once\n#include <string>\n\nusing namespace std;\n\nstring cat(string_view sv1, string_view sv2) {\n    string res(sv1.length()+sv2.length());\n    char* p = &res[0];\n\n    for (char c : sv1)                  // one way\n        *p++ = c;\n    copy(sv2.begin(), sv2.end(), p);    // another way\n    return res;\n}\n

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使用该Ctrl+F5命令会出现此错误。

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In file included from main.cpp:2:\nstrings.h: In function \xe2\x80\x98std::string cat(std::string_view, std::string_view)\xe2\x80\x99:\nstrings.h:7:41: error: no matching function for call to \xe2\x80\x98std::__cxx11::basic_string<char>::basic_string(std::basic_string_view<char>::size_type)\xe2\x80\x99\n    7 |     string res(sv1.length()+sv2.length());\n      |                                         ^\nIn file included from /usr/include/c++/11/string:55,\n                 from /usr/include/c++/11/bits/locale_classes.h:40,\n                 from /usr/include/c++/11/bits/ios_base.h:41,\n                 from /usr/include/c++/11/ios:42,\n                 from /usr/include/c++/11/ostream:38,\n                 from /usr/include/c++/11/iostream:39,\n                 from main.cpp:1:\n/usr/include/c++/11/bits/basic_string.h:650:9: note: candidate: \xe2\x80\x98template<class _Tp, class> std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _Tp&, const _Alloc&) [with _Tp = _Tp; <template-parameter-2-2> = <template-parameter-1-2>; _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]\xe2\x80\x99\n  650 |         basic_string(const _Tp& __t, const _Alloc& __a = _Alloc())\n      |         ^~~~~~~~~~~~\n/usr/include/c++/11/bits/basic_string.h:650:9: note:   template argument deduction/substitution failed:\nIn file included from /usr/include/c++/11/bits/move.h:57,\n                 from /usr/include/c++/11/bits/nested_exception.h:40,\n                 from /usr/include/c++/11/exception:148,\n                 from /usr/include/c++/11/ios:39,\n                 from /usr/include/c++/11/ostream:38,\n                 from /usr/include/c++/11/iostream:39,\n                 from main.cpp:1:\n

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我截取了从 Visual Studio 代码中看到的内容以指示带下划线的代码，这并不会使

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我认为这可能是代码编辑器的问题，但Godbolt显示了类似的错误。

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由于这与 Vs Code 有关，这里是我的

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launch.json

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{\n    // Use IntelliSense to learn about possible attributes.\n    // Hover to view descriptions of existing attributes.\n    // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387\n    "version": "0.2.0",\n    "configurations": [\n        {\n            "name": "g++-11 - Build and debug active file",\n            "type": "cppdbg",\n            "request": "launch",\n            "program": "${fileDirname}/${fileBasenameNoExtension}",\n            "args": [],\n            "stopAtEntry": false,\n            "cwd": "${fileDirname}",\n            "environment": [],\n            "externalConsole": false,\n            "MIMode": "gdb",\n            "setupCommands": [\n                {\n                    "description": "Enable pretty-printing for gdb",\n                    "text": "-enable-pretty-printing",\n                    "ignoreFailures": true\n                },\n                {\n                    "description": "Set Disassembly Flavor to Intel",\n                    "text": "-gdb-set disassembly-flavor intel",\n                    "ignoreFailures": true\n                }\n            ],\n            "preLaunchTask": "C/C++: g++-11 build active file",\n            "miDebuggerPath": "/usr/bin/gdb"\n        }\n    ]\n}\n

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和c_cpp_properties.json

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{\n    "configurations": [\n        {\n            "name": "Linux",\n            "includePath": [\n                "${workspaceFolder}/**"\n            ],\n            "defines": [],\n            "compilerPath": "/usr/bin/gcc",\n            "cStandard": "gnu17",\n            "cppStandard": "gnu++20",\n            "intelliSenseMode": "linux-gcc-x64"\n        }\n    ],\n    "version": 4\n}\n

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Answer 1

是的，这似乎是错误的。

线路

string res(sv1.length()+sv2.length());

应该

string res(sv1.length()+sv2.length(), '\0');

std::string与 相比，它没有仅接受大小参数的构造函数std::vector。必须显式提供用于初始化元素的值。

不过，我在这里的勘误表中找不到这个错误。