I'm stuck with a compilation problem. I do not understand why the following code does not compile:
#include <functional> namespace async { template<class ...T> using Callback = std::function<void(const std::string& result, T ...args)>; template<class Signature> class Function; template<class Res, class ...Args> class Function<Res(Args...)> { std::function<void(Args ...args, const Callback<Res>&)> m_function; public: Function(std::function<void(Args ...args, const Callback<Res>&)>&& function) : m_function(std::move(function)) {} Function& operator=(std::function<void(Args ...args, const Callback<Res>&)>&& function) {m_function = std::move(function); return *this;} void operator()(Args... args, const Callback<Res>& callback) const {m_function(args..., callback);} }; } async::Function<int(int)> getF() { return [](int i, const async::Callback<int> callback) { callback("", i); }; } int main(int argc, char** argv) { auto f0 = getF(); return 0; } gcc says:
In function ‘async::Function getF()’: error: could not convert ‘getF()::__lambda0{}’ from ‘getF()::__lambda0’ to ‘async::Function’
icc says:
error: no suitable user-defined conversion from "lambda [](int, async::Callback)->void" to "async::Function" exists
Now if I replace
return [](int i, const async::Callback<int> callback) { callback("", i); }; by
return async::Function<int(int)>([](int i, const async::Callback<int> callback) { callback("", i); }); Then it works. Why to I need to cast explicitely and how to avoid this?
A simpler solution would be to replace the class Function by something like
template<class Signature> using Function = std::nullptr_t; template<class Res, class ...Args> using Function<Res(Args...)> = std::function<void(Args ...args, const Callback<Res>&)>; But the specialization with <Res(Args...)> does not compile...
