您正在输出到ANSI终端吗？如果是这样，下面的转义序列应该有效：

#define underline "\033[4m"

More information on ANSI escape sequences is available here.
注意：要再次关闭下划线，请使用"\033[24m"。

也许最简单和最便携的方法就是这样：

cout << "This is the text which is going to be underlined." << endl;
cout << "-------------------------------------------------" << endl;

下面是一个为G++编写的扩展示例：

#include <iostream>
using namespace std;
int  main()
{
    char normal[]={0x1b,'[','0',';','3','9','m',0};
    char black[]={0x1b,'[','0',';','3','0','m',0};
    char red[]={0x1b,'[','0',';','3','1','m',0};
    char green[]={0x1b,'[','0',';','3', '2','m',0};
    char yellow[]={0x1b,'[','0',';','3', '3', 'm',0};
    char blue[]={0x1b,'[','0',';','3','4','m',0};
    char Upurple[]={0x1b,'[','4',';','3','5','m',0};
    char cyan[]={0x1b,'[','0',';','3','6','m',0};
    char lgray[]={0x1b,'[','0',';','3','7','m',0};
    char dgray[]={0x1b,'[','0',';','3','8','m',0};
    char Bred[]={0x1b,'[','1',';','3','1','m',0};
    //for bold colors, just change the 0 after the [ to a 1
    //for underlined colors, just change the 0 after the [ to a 4
    cout<<"This text is "<<black<<"Black "<<red<<"Red ";
    cout<<green<<"Green "<<yellow<<"Yellow "<<blue<<"Blue\n";
    cout<<Upurple<<"Underlined Purple "<<cyan<<"Cyan ";
    cout<<lgray<<"Light Gray "<<dgray<<"Dark Gray ";
    cout<<Bred<<"and Bold Red."<<normal<<"\n";
    return 0;
}

为了完成Paul R的回答，我有时会在控制台程序中创建以下函数：

std::string underline(const std::string &s) {
    return std::string(s.length(), '-');
}

然后您可以：

int main() {
    constexpr auto TEXT = "I am underlined";
    std::cout << TEXT << std::endl << underline(TEXT) << std::endl;
    return 0;
}

其他可能性：

void underlineAndDisplay(const std::string &s);
std::string underlineWith(const std::string &s, char c);

好吧，让我们回到我的代码...