std::function: Strict compile time verification of arguments
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28-10-2019 - |
Pergunta
I'd like to implement a class, which holds two callbacks with pre-defined function signatures.
The class has templated ctor, which uses std::bind to create std::function members. I expected that the compiler (g++ 4.6) would complain if a function with wrong signature is passed to the ctor. However, the compiler accepts the following:
callback c1(i, &test::func_a, &test::func_a);
I can understand why it does that. I tried to construct a proper condition for static_assert with no success.
How can I make a compile-time error to prevent this?
#include <functional>
using namespace std::placeholders;
class callback {
public:
typedef std::function<bool(const int&)> type_a;
typedef std::function<bool(int&)> type_b;
template <class O, typename CA, typename CB>
callback(O inst, CA ca, CB cb)
:
m_ca(std::bind(ca, inst, _1)),
m_cb(std::bind(cb, inst, _1))
{ }
private:
type_a m_ca;
type_b m_cb;
};
class test {
public:
bool func_a(const int& arg) { return true; }
bool func_b(int& arg) { arg = 10; return true; }
};
int main()
{
test i;
callback c(i, &test::func_a, &test::func_b);
// Both should fail at compile time
callback c1(i, &test::func_a, &test::func_a);
// callback c2(i, &test::func_b, &test::func_b);
return 0;
}
UPDATE: Answer from visitor solves my initial problem. Unfortunatelly I have a bunch of related cases to solve, which are demonstrated with the following code (http://ideone.com/P32sU):
class test {
public:
virtual bool func_a(const int& arg) { return true; }
virtual bool func_b(int& arg) { arg = 10; return true; }
};
class test_d : public test {
public:
virtual bool func_b(int& arg) { arg = 20; return true; }
};
int main()
{
test_d i;
callback c(i, &test_d::func_a, &test_d::func_b);
return 0;
}
static_assert as suggested by visitor is triggered here for this case, although the function signature is valid:
prog.cpp: In constructor 'callback::callback(O, CA, CB) [with O = test_d, CA = bool (test::*)(const int&), CB = bool (test_d::*)(int&)]':
prog.cpp:41:51: instantiated from here
prog.cpp:17:12: error: static assertion failed: "First function type incorrect"
I think it would be best just to compare function arguments and return value. Please suggest how.
Thank you.
Solução
You can statically assert in the constructor body:
static_assert(std::is_same<CA, bool(O::*)(const int&)>::value, "First function type incorrect");
static_assert(std::is_same<CB, bool(O::*)(int&)>::value, "Second function type incorrect");