为什么可以将nullptr分配给std :: string？ [英] Why can you assign nullptr to std::string?

问题描述

所以今天我写了一个很难发现的错误，其中我将std :: string初始化为nullptr（不是指向std :: string的指针，而是值本身）。我发现显然只有在C ++ 11或更高版本中才能使用clang。

So today I wrote a fairly hard to find bug where I initialized a std::string to nullptr (not a pointer to std::string, but the value itself). I've found apparently it's only possible to do in C++11 or later with clang.

#include <string>
#include <iostream>

using namespace std;
class Meh{
    int x;
};
class Foo
{
private:
  std::string x=nullptr;
  Meh y=nullptr; //remove this line and it compiles
public:
  std::string z=nullptr;
};

int main(void)
{
    Foo f;
    cout << f.z;
    return 0;
}

如您所见，我尝试将nullptr分配给a的随机实例类，它没有用。字符串中有什么魔术可以使它起作用，以及什至是有效的语法？我以为在这种情况下会遇到类型转换错误。

As you can see, I tried assigning nullptr to just a random instance of a class and it didn't work. What magic is in string that allows this to work, and in what way is this even valid syntax? I assumed I would be met with a type casting error in this case.

供参考，我使用以下命令进行编译：

For reference I compiled with this:

clang++ test.cpp -O3 -g -fno-inline -std=c++11 -Wall

它没有任何形式的警告，尽管如果不使用C ++ 11则会出错，

It gave no form of warnings, though it would error out if not using C++11

推荐答案

这仅仅是因为存在构造函数（链接中的数字（5））和赋值运算符（链接中的数字（3））用于 std :: string 接受 const char * ，因此 nullptr 匹配。

That's simply because there are constructors (number (5) in the link) and assignment operators (number (3) in the link) for std::string that accept a const char*, and hence the nullptr matches.

在C ++ 11之前（因此在 nullptr ）之前，当您尝试从 0 或进行构造时，会发生相同的问题。 NULL 。所有这些情况都是非法的，并导致不确定的行为，尽管至少一个STL（RogueWave？）过去接受了它并生成了一个空字符串。

Before C++11 (and therefore before nullptr), the same problem occurred when you tried to construct from 0 or NULL. All those cases were illegal and result in undefined behaviour, although at least one STL (RogueWave?) accepted it in the past and generated an empty string.

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