Boost Program Options中cout输出vector<string>时<<运算符报错

使用Visual Studio 2022、基于C++14标准开发时，按照Boost Program Options教程编写了以下代码：

#include <iostream>
#include <vector>
#include <boost/program_options.hpp>

namespace po = boost::program_options;

int main(int argc, char* argv[]) {
    po::options_description desc("Allowed options");
    desc.add_options()
        ("include-path,I", po::value<std::vector<std::string>>()->multitoken(), "Set include paths");

    po::variables_map vm;
    po::store(po::parse_command_line(argc, argv, desc), vm);
    po::notify(vm);

    if (vm.count("include-path"))
    {
        std::cout << "Include paths are: "
            << vm["include-path"].as<std::vector<std::string>>() << "\n";
    }

    return 0;
}

编译时第二个<<运算符报错，提示不存在匹配操作数的<<运算符。

问题原因 #

C++标准库并未为std::vector提供默认的std::ostream输出运算符重载，因此直接将std::vector<std::string>对象传入std::cout会触发编译错误。

解决方案 #

方案1：手动遍历输出vector元素

最稳妥且符合标准的方式是手动遍历vector并输出每个元素，无需依赖自定义运算符：

if (vm.count("include-path"))
{
    const auto& include_paths = vm["include-path"].as<std::vector<std::string>>();
    std::cout << "Include paths are: [";
    for (size_t i = 0; i < include_paths.size(); ++i)
    {
        if (i != 0)
            std::cout << ", ";
        std::cout << include_paths[i];
    }
    std::cout << "]\n";
}

方案2：修正自定义运算符重载

如果你坚持使用自定义运算符重载，需要确保编译器能正确找到该重载。由于std::vector位于std命名空间，可将重载放入std命名空间（注：此操作虽在多数编译器中可行，但不符合C++标准对扩展std命名空间的规定，需谨慎使用）：

#include <iostream>
#include <vector>

namespace std {
    template <class T>
    std::ostream& operator <<(std::ostream& os, const std::vector<T>& v) {
        os << "[";
        for (auto it = v.begin(); it != v.end(); ++it) {
            if (it != v.begin())
                os << ", ";
            os << *it;
        }
        os << "]";
        return os;
    }
}

或者在输出时显式调用全局命名空间的重载：

if (vm.count("include-path"))
{
    std::cout << "Include paths are: ";
    ::operator<<(std::cout, vm["include-path"].as<std::vector<std::string>>()) << "\n";
}

方案3：使用Boost Range辅助输出

借助Boost Range的遍历功能简化输出（需包含<boost/range/algorithm/for_each.hpp>）：

#include <boost/range/algorithm/for_each.hpp>

// ...

if (vm.count("include-path"))
{
    const auto& include_paths = vm["include-path"].as<std::vector<std::string>>();
    std::cout << "Include paths are: [";
    bool first = true;
    boost::for_each(include_paths, [&](const std::string& path) {
        if (!first)
            std::cout << ", ";
        std::cout << path;
        first = false;
    });
    std::cout << "]\n";
}

内容的提问来源于stack exchange，提问作者DanXIV