C++ 字符串流内联 [英] C++ stringstream inline

问题描述

我想使用 std::stringstream 来创建格式化的字符串，但使用内联类，所以我没有 stringstream 局部变量四处乱飞.我的意思是:

I would like to use std::stringstream to create formatted strings, but use the class inline so I don't have stringstream local variables flying around. What I mean is this:

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

int main(int argc, char* argv[])
{
    std::string test = ((std::ostringstream&)
         (std::ostringstream("") << "This is a test: " << 50.1 << "abc")
         ).str();
    std::cout << test << std::endl;
    return 0;
}

这在 GCC 中编译得很好，但是输出如下:

This compiles fine in GCC, however the output is the following:

"0x401d0a50.1abc"

"0x401d0a50.1abc"

所以看起来stringstream把第一个字符串当成一个指针，输出地址.后续 operator<< 的工作正常.

So it seems that stringstream treats the first string as a pointer and outputs the address. Subsequent operator<<'s work fine.

我该如何解决这个问题?谢谢！

How do I fix this? Thanks!

推荐答案

原因是 << 运算符是 void const* 的成员，但是一个自由函数，将 std::ostream& 作为 char const* 的左侧参数.你的 std::ostringstream( "" ) 是一个临时的:你可以在它上面调用成员函数(甚至非常量成员函数)，但临时不能用于初始化非常量引用全局函数.

The reason is that the << operator is a member for void const*, but a free function taking an std::ostream& as the left hand argument for char const*. Your std::ostringstream( "" ) is a temporary: you can call member functions (even non-const member functions) on it, but a temporary cannot be used to initialize the non-const reference of the global function.

两点:首先，正如已经指出的，g++ 确实做你想做的事如果您指定 -std=c++11，则需要.作为 T.C.指出，这是在 §27.7.3.9 中指定，它为所有 << 带有 std::istream 的右值引用范围.其次，经典的解决方法是开始表达式 std::ostringstream( "" ).flush() <<....flush 是一个成员函数(因此可以在一个临时的)，它返回一个 std::ostream&(所以一切否则很好地链接)；它也没有任何作用一个 std::ostringstream.

Two points: first, as has been pointed out, g++ does do what you want if you specify -std=c++11. As T.C. points out, this is specified in §27.7.3.9, which provides a template overload for all << with an rvalue reference for the std::istream parameter. And second, the classic work around is to start the expression std::ostringstream( "" ).flush() <<.... flush is a member function (and so can be called on a temporary) which returns an std::ostream& (so everything else chains nicely); it also does nothing on a std::ostringstream.

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