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[英]How to write Wrapper for accessing C++ class member from C (with inheritance and constructor)
[英]how to write a C++ class member function wrapper?
我想包装一些类成员函数,并围绕它们做一些准备和清理工作。
我尝试复制其他一些线程池代码,但出现一些我无法处理的错误。 如何正确做?
#include <iostream>
#include <string>
using namespace std;
class A {
public:
void connect() {};
void close() {};
template<typename F, typename ... Args>
auto wrapper(F&& f, Args&& ... args) -> typename std::result_of<F(Args...)>::type {
using return_type = typename std::result_of<F(Args...)>::type;
connect();
return_type ret = f(args...);
close();
return ret;
}
bool c(int a, string b) {}
string c(string b) {return b;}
bool r(int a, string b) {}
};
int main() {
A a;
a.connect();
a.c(1, "abc");
a.close(); // equal to a.wrapper(a.c, 1, "abc"); but compling error, how to write it correctly?
cout << "result of a is: " << a.wrapper(a.c, 1, "abc") ? "successful" : "fail" << endl;
cout << "result of another a is: " << a.wrapper(a.c, "abc") << endl;
cout << "result of r is:" << a.wrapper(a.r, 1, "abc") << endl;
}
我得到这样的错误:
main.cpp: In function ‘int main()’:
main.cpp:25:58: error: no matching function for call to ‘A::wrapper(<unresolved overloaded function type>, int, const char [4])’
cout << "result of a is: " << a.wrapper(a.c, 1, "abc") ? "successful" : "fail" << endl;
^
main.cpp:25:58: note: candidate is:
main.cpp:9:10: note: template<class F, class ... Args> typename std::result_of<_Functor(_ArgTypes ...)>::type A::wrapper(F&&, Args&& ...)
auto wrapper(F&& f, Args&& ... args) -> typename std::result_of<F(Args...)>::type {
^
main.cpp:9:10: note: template argument deduction/substitution failed:
main.cpp:25:58: note: couldn't deduce template parameter ‘F’
cout << "result of a is: " << a.wrapper(a.c, 1, "abc") ? "successful" : "fail" << endl;
^
main.cpp:25:87: error: invalid operands of types ‘const char [5]’ and ‘<unresolved overloaded function type>’ to binary ‘operator<<’
cout << "result of a is: " << a.wrapper(a.c, 1, "abc") ? "successful" : "fail" << endl;
^
main.cpp:26:63: error: no matching function for call to ‘A::wrapper(<unresolved overloaded function type>, const char [4])’
cout << "result of another a is: " << a.wrapper(a.c, "abc") << endl;
^
main.cpp:26:63: note: candidate is:
main.cpp:9:10: note: template<class F, class ... Args> typename std::result_of<_Functor(_ArgTypes ...)>::type A::wrapper(F&&, Args&& ...)
auto wrapper(F&& f, Args&& ... args) -> typename std::result_of<F(Args...)>::type {
^
main.cpp:9:10: note: template argument deduction/substitution failed:
main.cpp:26:63: note: couldn't deduce template parameter ‘F’
cout << "result of another a is: " << a.wrapper(a.c, "abc") << endl;
^
后面的代码将通过编译。 但是,它仍然比预期的复杂得多。
using sig1 = bool(A::*)(int, string);
cout << "result of a is: " << (a.wrapper(bind(static_cast<sig1>(&A::c), a, _1, _2), 1, string("abc")) ? "successful" : "fail") << endl;
//cout << "result of another a is: " << a.wrapper(a.c, "abc") << endl;
//cout << "result of r" << a.wrapper(a.r, 1, "abc") << endl;
顺便说一句,clang的错误报告比gcc更好。 当您在某些模板编程方面遇到麻烦时,您想获得更清晰的错误消息,仅供参考
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