// This can be an ofstream as well or any other ostream
std::stringstream buffer;
// Save cout's buffer here
std::streambuf *sbuf = std::cout.rdbuf();
// Redirect cout to our stringstream buffer or any other ostream
std::cout.rdbuf(buffer.rdbuf());
// Use cout as usual
std::cout << "Hello World";
// When done redirect cout to its old self
std::cout.rdbuf(sbuf);
参考
https://stackoverflow.com/questions/3803465/how-to-capture-stdout-stderr-with-googletest