C++ 标准的单元测试

Question

I'm trying to make unit tests with criterion for my C++ code but I can't figure out how to test a function that only print and do not return anything. Here's what I tried:

//the function to test

#include <iostream>
#include <fstream>

void my_cat(int ac, char **av)
{
    if (ac <= 1)
        std::cout << "my_cat: Usage: ./my_cat file [...]" << std::endl;
    for (unsigned i = 1; i < ac; i += 1) {
        std::ifstream file (av[i]);
        if (file.fail()) {
            std::cout << "my_cat: ";
            std::cout << av[i];
            std::cout << ": No such file or directory" << std::endl;
        }
        else if (file.is_open()) {
            std::cout << file.rdbuf() << std::endl;
        }
        file.close();
    }
}

//the test
#include  <criterion/criterion.h>
#include  <criterion/redirect.h>

void my_cat(int ac, char **av);

Test(mycat, my_cat)
{
    char *av[] = {"./my_cat", "text.txt"};
    my_cat(2, av);
}

But now that I'm here I don't know what to use to check if the print is correct.

Answer 1

With gtest, I think this can help you

testing::internal::CaptureStdout();
std::cout << "My test";
std::string output = testing::internal::GetCapturedStdout();

refer from: How to capture stdout/stderr with googletest?

Answer 2

The other answer shows how to use a googletest facility. However, in general when your code is difficult to test, then that is a code smell. Consider this simpler example:

void foo(){
    std::cout << "hello";
}

This is much easier to test when don't use std::cout directly, but pass the stream to be used as parameter:

#include <iostream>
#include <sstream>

void foo(std::ostream& out){
    out << "hello";
}
    
int main() {
    std::stringstream ss;
    foo(ss);
    std::cout << (ss.str() == "hello");
}

In general, I do not recommend to use std::cout directly for anything but small toy programs. You never know if later you want to write to a file or some other stream.