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本文最后更新于 2024-07-21，文中内容可能已过时。
通过 traits 把 lambda 转化成函数指针。
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// Source: https://stackoverflow.com/a/48368508/17132546
#include <iostream>
using namespace std;
// Entry template
// extract the lambda's operaor() function signature
template <class F, class T=F>
struct lambda_traits: lambda_traits<decltype(&std::remove_reference<F>::type::operator()), F>
{};
// For mutable lambda, See https://en.cppreference.com/w/cpp/language/lambda
// mutable lambda's operator() is not const,
// not mutable lambda's operator() is const
template <typename rF, typename F, typename R, typename... Args>
struct lambda_traits<R(rF::*)(Args...), F>: lambda_traits<R(rF::*)(Args...) const, F>
{};
// Workhorse
// every lambda has an unique signature
// => lambda_traits will be specialized for every lambda, even if their function signature are the same.
template <typename rF, typename F, typename R, typename... Args>
struct lambda_traits<R(rF::*)(Args...) const, F> {
static auto cify(F&& f) {
static rF fn = std::forward<F>(f);
return [](Args... args) {
return fn(std::forward<Args>(args)...);
};
}
};
// Wrapper, for convenience
template <class F>
inline auto lam2fp(F&& f) {
return lambda_traits<F>::cify(std::forward<F>(f));
}
// usage
class A {
public:
using cb_t = void(*)(int);
A() { cout << "A()\n"; }
A(const A& k) { cout << "A(const A&)\n"; }
A(A&& k) { cout << "A(const A&&)\n"; }
void register_cb(cb_t cb)
{
_cb = cb;
}
void run()
{
if (_cb)
{
_cb(_i++);
}
}
private:
int _i {0};
cb_t _cb {nullptr};
};
struct X
{
X()
{
cout << "X::ctor" << endl;
}
int a {0};
};
int main() {
{
A a;
X x;
cout << "register_cb:-------------------" << endl;
a.register_cb(lam2fp([&x](int i)
{
cout << "i:" << endl;
cout << "x.a:" << x.a << endl;
})
);
cout << "run1:" << endl;
a.run();
cout << "run2:" << endl;
a.run();
}
#if 0
auto lam_left = [&](A& a) {};
auto lam_copy = [](A a) {};
// lambda: left ref args: left ref
auto g_left = lam2fp(lam_left);
// lambda: left ref args: copy -> right ref
auto g = lam2fp(lam_copy);
// lambda: right ref args: right ref
auto g_right = lam2fp([](A&& a) {});
cout << "----g_left----\n";
A a;
g_left(a);
cout << "----g----\n";
A b;
g(b);
cout << "----g_right----\n";
g_right(A());
#endif
return 0;
}
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// Source: https://stackoverflow.com/a/48368508/17132546
#include <iostream>
using namespace std;
// Entry template
// extract the lambda's operaor() function signature
template <class F, class T=F>
struct lambda_traits: lambda_traits<decltype(&std::remove_reference<F>::type::operator()), F>
{};
// For mutable lambda, See https://en.cppreference.com/w/cpp/language/lambda
// mutable lambda's operator() is not const,
// not mutable lambda's operator() is const
template <typename rF, typename F, typename R, typename... Args>
struct lambda_traits<R(rF::*)(Args...), F>: lambda_traits<R(rF::*)(Args...) const, F>
{};
// Workhorse
// every lambda has an unique signature
// => lambda_traits will be specialized for every lambda, even if their function signature are the same.
template <typename rF, typename F, typename R, typename... Args>
struct lambda_traits<R(rF::*)(Args...) const, F> {
static auto cify(F&& f) {
static rF fn = std::forward<F>(f);
return [](Args... args) {
return fn(std::forward<Args>(args)...);
};
}
};
// Wrapper, for convenience
template <class F>
inline auto lam2fp(F&& f) {
return lambda_traits<F>::cify(std::forward<F>(f));
}
// usage
class A {
public:
using cb_t = void(*)(int);
A() { cout << "A()\n"; }
A(const A& k) { cout << "A(const A&)\n"; }
A(A&& k) { cout << "A(const A&&)\n"; }
void run(void(*cb)(int))
{
cb(_i++);
}
private:
int _i {0};
};
struct X
{
X()
{
cout << "X::ctor" << endl;
}
int a {0};
};
int main() {
{
A a;
X x;
cout << "start:-------------------" << endl;
cout << "run1:" << endl;
a.run(lam2fp([&x](int i)
{
cout << "i:" << endl;
cout << "x.a:" << x.a << endl;
}));
cout << "run1:" << endl;
a.run(lam2fp([&x](int i)
{
cout << "i:" << endl;
cout << "x.a:" << x.a << endl;
}));
}
#if 0
auto lam_left = [&](A& a) {};
auto lam_copy = [](A a) {};
// lambda: left ref args: left ref
auto g_left = lam2fp(lam_left);
// lambda: left ref args: copy -> right ref
auto g = lam2fp(lam_copy);
// lambda: right ref args: right ref
auto g_right = lam2fp([](A&& a) {});
cout << "----g_left----\n";
A a;
g_left(a);
cout << "----g----\n";
A b;
g(b);
cout << "----g_right----\n";
g_right(A());
#endif
return 0;
}
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