Verify content of nlohmann json which is member of mocked method argument - c++

Question

Assume I got a some Message class object which has nlohmann json as private member and public getter:
class Message{
public:
nlohmann::json getJson() { return json_;}
...
private:
nlohmann::json json_;
...
};
Also there is a class that publishes the message
ex:
class Foo {
public:
publish(const Message& message)
...
};
In the test I am mocking the Foo::publish method and in some scenario I want to check if json_["key1"]["key2"] value is different than "" (empty string)
EXPECT_CALL(
*foo_mock_pointer,
publish(x) // x is the unknown code
);
For checking the value of the json object I guess it will be enough:
testing::Contains(testing::Pair("key1", testing::Pair("key2"), testing::Ne("")))
But I cant figure out how to get the json from Message object which is the argument of the mocked method.

Answer 1

IIUC, it looks like you want to check something about the argument that is passed to your mock function.
You can use SaveArg to save that argument inside a variable and then check its value later:
Message message;
EXPECT_CALL(
*foo_mock_pointer,
publish(x) // x is the unknown code
).WillOnce(DoAll(SaveArg<0>(&message), Return(/*Whatever you want to return*/)));
// Call your class-under-test API here
// ...
// Now check the message:
EXPECT_THAT(message.getJson(), /*Insert your matcher here*/);
See here for more info: http://google.github.io/googletest/gmock_cook_book.html#SaveArgVerify