How to mock an external library in tests (httplib::Client )

Generally when writing unit tests, we create classes from interfaces and create mock classes for tests. e.g.

class TestedClass {

   TestedClass(std::shared_ptr<IDependency> dependency_ptr){...}

}

So it is easy to mock the dependency here by just inheriting from IDependency, like:

class MockDependency: public IDependency {
...
}

But how do we follow that approach for third party libraries? In this particular case, I am using httplib::Client. Since it is a concrete class from a thirdparty vendor, how can I mock that to test my own class which uses this class (injected via dependency injection)?

I doubt you are using every method httplib::Client can offer, so try to design your own interface that is enough for your application, e.g.:

class IHttpClient {
public:
    virtual ~IHttpClient() = default;
    virtual std::string get(std::string url) = 0;
    // more interfaces
};

Then your production implementation can be e.g. like this:

    std::string get(std::string url) override {
        httplib::Client cli(url);
        auto res = cli.Get("/hi");
        // error handling, maybe throw something
        return res->body;
    }

and in the tests you can have

ON_CALL(mockClient, get(matchingUrl)).WillByDefault(Return("expected reply"));