c++ - How to unit test function writing to stdout / std::cout -

these days, studying unit test. unit test use return value or reference value expected value in test case. if has not return value , reference value in function. expected value?

exsample-

void unit_test() {   cout << "hello" << endl; } 

sure, unit_test function simple. so, function not seem need unit test. but, sample. think unit_test function has side-effect.

thank you, , please understand fool english.

if writing function know should tested, should design testable in framework. here, if testing done @ process level can verify process output, writing std::cout fine. otherwise, may want make output stream parameter function, in:

void unit_test(std::ostream& os = std::cout)  {    os << "hello" << endl;  }  

then can test in:

std::ostringstream oss; unit_test(oss); assert(oss && oss.str() == "hello"); 

as illustrates, making well-tested software requires bit of give , take... testing requirements feed design.

edit: if must test pre-existing functions without changing them, consider:

#include <sstream> #include <iostream>  void f() {     std::cout << "hello world\n"; }  int main() {     std::ostringstream oss;     std::streambuf* p_cout_streambuf = std::cout.rdbuf();     std::cout.rdbuf(oss.rdbuf());      f();      std::cout.rdbuf(p_cout_streambuf); // restore      // test oss content...     assert(oss && oss.str() == "hello world\n";     std::cout << oss.str(); }