CODEWITHSUNDEEP
c++templateslambda
4LegsDrivenCat
4LegsDrivenCat

Reputation: 1361

Check function pointer for nullptr in a template for any kind of callable

I created simple wrapper to catch, report and rethrow exceptions (see below). It works fine for functions, function pointers and std::function objects but fails to compile for lambdas and functors due to the check for nullptr. Is there a way to fix this as simply as possible so that wrapper could be used for any kind of callable? Thanks!

#include <functional>

template<typename Func, typename TRet, typename... Args>
TRet wrapper(Func func, TRet exit_code_on_error, Args... args) {
    TRet exit_code = exit_code_on_error;
    //if (func) // this condition does not compile for lambdas and functors
    {
        try {
            exit_code = func(std::forward<Args>(args)...);
        } catch(...) {
            // report and possibly rethrow
            //throw;
        }
    }
    return exit_code;
}

int test1(double d) {
    return (int)d;
}

int test2(std::function<int (double)> f, double d) {
    return f(d);
}

struct TestFunctor {
    int operator()(double d) {
        return (int)d;
    }
};

int main() {
    // OK:
    wrapper(test1, 1, 2.3);
    wrapper(&test1, 1, 2.3);
    auto p = test1;
    wrapper(p, 1, 2.3);
    p = nullptr;
    wrapper(p, 1, 2.3);
    wrapper(test2, 1, test1, 2.3);

    // These lines cause the troubles:
    wrapper([](double d){ return (int)d; }, 1, 2.3);
    wrapper(TestFunctor(), 1, 2.3);
}

Error:

wrapper.hpp: error C2451: conditional expression of type 'Func' is illegal
        with
        [
            Func=main::<lambda_1>
        ]

Upvotes: 1

Views: 683

Answers (1)

NathanOliver
NathanOliver

Reputation: 180500

We can fix this by first checking if Func is a pointer type and then if it is, check if it is null pointer. That would look like

template<typename Func, typename TRet, typename... Args>
TRet wrapper(Func func, TRet exit_code_on_error, Args... args) {
    if constexpr(std::is_pointer_v<Func> ||
                 std::is_member_function_pointer_v<Func> ||
                 std::is_same_v<Func, std::function<TRet(Args...)>>) {
        // this will only be compiled if Func is a pointer type or std::function
        if (func == nullptr)
            return exit_code_on_error;
    }
    try {
        return func(std::forward<Args>(args)...);
    } catch(...) {
        // report and rethrow if needed
        //throw;
    }
    return exit_code_on_error;
}

Upvotes: 5

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