How to test boost logger output in unit testing?

Question

My application uses BOOST_LOG_TRIVIAL to log messages. I thought it's good to check in test (gtest) that correct messages are written there in given scenarios.

Is there a way to somehow access what was written there after code was called? Or does it first have to be mocked in some way?

I have googled a lot for this and haven't found any instructions, so either I am asking wrong questions or no one thinks it should be done.

Answer 1

In your googletest suite, you can use the boost::log facilities in each test case to redirect the BOOST_LOG_TRIVIAL messages to a file. After writing the BOOST_LOG_TRIVIAL message(s) you want you can then flush the file, open it, and check that it has the contents you expect. For example:

gtester.cpp

#include <gtest/gtest.h>
#include <boost/shared_ptr.hpp>
#include <boost/log/sinks/sync_frontend.hpp>
#include <boost/log/sinks/text_file_backend.hpp>
#include <boost/log/utility/setup/file.hpp>
#include <boost/log/trivial.hpp>
#include <string>
#include <fstream>

using sink_t = boost::log::sinks::synchronous_sink<boost::log::sinks::text_file_backend>;

struct boost_log_tester : ::testing::Test {
    void SetUp() {
        file_sink = boost::log::add_file_log("boost.log");
    }
    void TearDown() {
        boost::log::core::get()->remove_sink(file_sink);
        file_sink.reset();
    }
protected:
    boost::shared_ptr<sink_t> file_sink;
};

TEST_F(boost_log_tester,info_msg)
{
    std::string msg = "An informational severity message";
    BOOST_LOG_TRIVIAL(info) << msg;
    file_sink->flush();
    std::ifstream captured_cout("boost.log");
    ASSERT_TRUE(captured_cout.good()) << "Failure executing test: Could not open `boost.log` for reading";
    std::string cout_str;
    std::getline(captured_cout,cout_str);
    EXPECT_NE(cout_str.find(msg),std::string::npos);
}

TEST_F(boost_log_tester,error_msg)
{
    std::string msg = "An error severity message";
    BOOST_LOG_TRIVIAL(error) << msg;
    file_sink->flush();
    std::ifstream captured_cerr("boost.log");
    ASSERT_TRUE(captured_cerr.good()) << "Failure executing test: Could not open `boost.log` for reading";
    std::string cerr_str;
    std::getline(captured_cerr,cerr_str);
    EXPECT_NE(cerr_str.find(msg),std::string::npos);
}

int main(int argc, char **argv) {
    ::testing::InitGoogleTest(&argc, argv);
    return RUN_ALL_TESTS();
}

Compile and link:

$ g++ -Wall -Wextra -DBOOST_LOG_DYN_LINK -c gtester.cpp
$ g++ -o gtester gtester.o -lboost_log -lboost_thread -lboost_system -lgtest -pthread

And it runs like:

$ ./gtester 
[==========] Running 2 tests from 1 test case.
[----------] Global test environment set-up.
[----------] 2 tests from boost_log_tester
[ RUN      ] boost_log_tester.info_msg
[       OK ] boost_log_tester.info_msg (0 ms)
[ RUN      ] boost_log_tester.error_msg
[       OK ] boost_log_tester.error_msg (2 ms)
[----------] 2 tests from boost_log_tester (2 ms total)

[----------] Global test environment tear-down
[==========] 2 tests from 1 test case ran. (2 ms total)
[  PASSED  ] 2 tests.