为什么C ++ regex_match函数要求在该函数之外定义搜索字符串? [英] Why does the c++ regex_match function require the search string to be defined outside of the function?

问题描述

我正在使用Visual Studio 2013进行开发，该版本使用Microsoft C ++编译器工具的v12.
以下代码可以正常执行，将"foo"打印到控制台:

I am using Visual Studio 2013 for development, which uses v12 of Microsoft's c++ compiler tools.
The following code executes fine, printing "foo" to the console:

#include <regex>
#include <iostream>
#include <string>

std::string get() {
    return std::string("foo bar");
}

int main() {
    std::smatch matches;
    std::string s = get();
    std::regex_match(s, matches, std::regex("^(foo).*"));
    std::cout << matches[1] << std::endl;
}
// Works as expected.

相同的代码，用字符串"s"代替"get()"函数，在运行时抛出字符串迭代器不兼容"错误:

The same code, with the string "s" substituted for the "get()" function, throws a "string iterators incompatible" error at runtime:

#include <regex>
#include <iostream>
#include <string>

std::string get() {
    return std::string("foo bar");
}

int main() {
    std::smatch matches;
    std::regex_match(get(), matches, std::regex("^(foo).*"));
    std::cout << matches[1] << std::endl;
}
// Debug Assertion Failed!
// Expression: string iterators incompatible

这对我来说毫无意义.谁能解释为什么会这样?

This makes no sense to me. Can anyone explain why this happens?

推荐答案

原因是 get()返回一个临时字符串，因此匹配结果将迭代器包含到不再存在的对象中，尝试使用它们是不确定的行为.Visual Studio C ++库中的调试声明会注意到此问题并中止您的程序.

The reason is that get() returns a temporary string, so the match results contains iterators into an object that no longer exists, and trying to use them is undefined behaviour. The debugging assertions in the Visual Studio C++ library notice this problem and abort your program.

最初，C ++ 11确实允许您尝试执行操作，但是由于它是如此危险，因此通过添加已删除的 std :: regex_match 重载来防止它被阻止，该重载在尝试执行时会被使用.要从临时字符串中获取匹配结果，请参见 LWG DR 2329 .这意味着您的程序不应在C ++ 14中编译(或在以C ++ 11模式实现DR的编译器中).GCC尚未实施更改，我会解决.

Originally C++11 did allow what you're trying to do, but because it is so dangerous it was prevented by adding a deleted overload of std::regex_match which gets used when trying to get match results from a temporary string, see LWG DR 2329. That means your program should not compile in C++14 (or in compilers that implement the DR in C++11 mode too). GCC does not yet implement the change yet, I'll fix that.

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