如何在C++17中结合变量正确格式化输出字符串？

Great question! Since C17 doesn’t ship with std::format built-in, you don’t have to settle for splitting strings and variables across multiple output calls. There are several clean, maintainable alternatives that match the elegance of the C20 approach. Here are the most practical options:

1. 使用标准库的std::ostringstream #

This is the go-to standard library solution—no external dependencies, and it uses the familiar << operator syntax you already know from std::cout. You can build your formatted string in a stream, then extract it as a std::string when you’re done:

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

int main() {
    int a = 13;
    int b = 50;
    
    std::ostringstream oss;
    oss << "The respondents of the survey were " << a << "-" << b << " years old.";
    
    std::string out_str = oss.str();
    std::cout << out_str << std::endl;
    return 0;
}

It works with any type that supports the insertion operator (<<), so you can format numbers, strings, custom types, and more without extra work.

2. 使用fmtlib（std::format的前身） #

If you’re open to a lightweight third-party library, fmtlib is the perfect choice. It’s the library that inspired C20’s std::format, so its syntax is identical—meaning you can easily switch to the standard std::format later if you move to C20.

First, install the fmt library (it’s available via package managers like Conan, vcpkg, or you can build it from source). Then use it like this:

#include <iostream>
#include <string>
#include "fmt/core.h"

int main() {
    int a = 13;
    int b = 50;
    
    std::string out_str = fmt::format("The respondents of the survey were {}-{} years old.", a, b);
    std::cout << out_str << std::endl;
    return 0;
}

Fmtlib is fast, type-safe, and supports advanced formatting options (like padding, precision, and custom format specifiers) that std::ostringstream doesn’t handle as cleanly.

3. 简化直接输出（无需先构建字符串） #

If you don’t need to store the formatted string and just want to print it directly, you can chain all your output operations in a single line—no need to split them into multiple std::cout calls:

#include <iostream>

int main() {
    int a = 13;
    int b = 50;
    
    std::cout << "The respondents of the survey were " << a << "-" << b << " years old." << std::endl;
    return 0;
}

This is simpler than building a string first if your only goal is to print to the console.

总结 #

• Stick with std::ostringstream for a dependency-free, standard solution.
• Use fmtlib if you want the exact syntax of C20’s std::format (and better performance/features) in C17.
• Chain std::cout calls for quick, direct output without string building.

内容的提问来源于stack exchange，提问作者Jack Avante