How to do unit test for this?

how do i create a unit test for a function like this?

void TextBuddy::addSentence(char *argv[]){

   file.push_back(content);
   updateFile(argv);
   sprintf_s(message, ADD_MSG.c_str(), argv[1], content.c_str());
   output(message);

return;

i'm unable to create a unit test for this function even after trying many many times...

namespace Tester
{
TEST_CLASS(UnitTest)
{
public:
        Textbuddy test; 
    TEST_METHOD(addtest)
    {   
        test.addSentence("hello")
        std::string result = "hello";
        Assert::AreEqual("hello",result);