std::ostringstream 打印 c 字符串的地址而不是其内容

2022-01-07 00:00:00 stream c++

我偶然发现了一个我一开始无法解释的奇怪行为(参见 ideone):

I have stumbled on a weird behavior that I just could not explain at first (see ideone):

#include <iostream>
#include <sstream>
#include <string>

int main() {
  std::cout << "Reference     : "
            << (void const*)"some data"
            << "
";

  std::ostringstream s;
  s << "some data";
  std::cout << "Regular Syntax: " << s.str() << "
";

  std::ostringstream s2;
  std::cout << "Semi inline   : "
            << static_cast<std::ostringstream&>(s2 << "some data").str()
            << "
";

  std::cout << "Inline        : "
            << dynamic_cast<std::ostringstream&>(
                 std::ostringstream() << "some data"
               ).str()
            << "
";
}

给出输出:

Reference     : 0x804a03d
Regular Syntax: some data
Semi inline   : some data
Inline        : 0x804a03d

令人惊讶的是，在最后一次演员表中，我们有地址，而不是内容！

Surprisingly, in the last cast we have the address, and not the content!

为什么会这样?

推荐答案

表达式std::ostringstream() 创建一个临时的，operator<<< 需要const char* as argument 是一个自由函数，但是这个自由函数不能临时调用，因为函数的第一个参数的类型是std::ostream& 不能绑定到临时对象.

The expressionstd::ostringstream() creates a temporary, and operator<< which takes const char* as argument is a free function, but this free function cannot be called on a temporary, as the type of the first parameter of the function is std::ostream& which cannot be bound to temporary object.

话虽如此，<<std::ostringstream() <<一些数据" 解析为对打印地址的 void* 重载的成员函数的调用.请注意，可以在临时上调用成员函数.

Having said that, <<std::ostringstream() << "some data" resolves to a call to a member function which is overloaded for void* which prints the address. Note that a member function can be invoked on the temporary.

为了调用自由函数，您需要将临时(即右值)转换为左值，这是您可以执行的一个技巧:

In order to call the free function, you need to convert temporary (which is rvalue) into a lvalue, and here is one trick that you can do:

 std::cout << "Inline        : "
            << dynamic_cast<std::ostringstream&>(
                 std::ostringstream().flush() << "some data"
               ).str()
            << "
";

即 std::ostringstream().flush() 返回 std::ostream& 这意味着，现在可以调用自由函数，传递返回的引用作为第一个论点.

That is, std::ostringstream().flush() returns std::ostream& which means, now the free function can called, passing the returned reference as first argument.

另外，你不需要在这里使用 dynamic_cast(这很慢，因为它是在运行时完成的)，因为对象的类型是非常已知的，所以你可以使用static_cast(它在编译时完成得很快):

Also, you don't need to use dynamic_cast here (which is slow, as it is done at runtime), for the type of the object is pretty much known, and so you can use static_cast (which is fast as it is done at compile-time):

 std::cout << "Inline        : "
            << static_cast<std::ostringstream&>(
                 std::ostringstream().flush() << "some data"
               ).str()
            << "
";

应该可以正常工作.

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