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#ifndef BASE_FUNCTION_REF_H_
#define BASE_FUNCTION_REF_H_
#include "config.h"
#include <type_traits>
#include <utility>
namespace tcmalloc {
template <typename T>
struct FunctionRef;
// FunctionRef is to std::function what std::string_view is to
// std::string. That is non-owning, trivially copyable reference-like
// type to anything that can be invoked. It is mostly equivalent to
// absl::FunctionRef. Just like std::string_view we encourage people
// to pass it by value and in most most modern ABIs it will be
// efficiently passed in two registers.
//
// It avoids any memory allocation (unlike std::function). So good to
// use everywhere even inside tightest guts of malloc or
// super-early. Of course, since it doesn't own it's invokable, it
// must not outlive it.
//
// Our version also explicitly exposes pure C-style function and data
// arguments to be used by legacy C-style API's that pass callback
// function and callback argument in order to implement closure-like
// features.
template <typename R, typename... Args>
struct FunctionRef<R(Args...)> {
template <typename F,
typename FR = std::invoke_result_t<F, Args&&...>>
using EnableIfCompatible =
typename std::enable_if<std::is_void<R>::value ||
std::is_convertible<FR, R>::value>::type;
template <typename Body, typename = EnableIfCompatible<const Body&>>
FunctionRef(const Body& body)
: fn([] (Args... args, void* data) {
const Body& b = *(reinterpret_cast<const Body*>(data));
return b(std::forward<Args>(args)...);
}), data(const_cast<void*>(reinterpret_cast<const void*>(&body))) {}
R operator()(Args... args) const {
return fn(std::forward<Args>(args)..., data);
}
R (*const fn)(Args... args, void* data);
void* const data;
};
template <typename T>
struct FunctionRefFirstDataArg;
// FunctionRefFirstDataArg is same as FunctionRef except it's fn's
// function type accepts data argument in first position.
template <typename R, typename... Args>
struct FunctionRefFirstDataArg<R(Args...)> {
template <typename F,
typename FR = std::invoke_result_t<F, Args&&...>>
using EnableIfCompatible =
typename std::enable_if<std::is_void<R>::value ||
std::is_convertible<FR, R>::value>::type;
template <typename Body>
FunctionRefFirstDataArg(const Body& body)
: fn([] (void* data, Args... args) {
const Body& b = *(reinterpret_cast<const Body*>(data));
return b(std::forward<Args>(args)...);
}), data(const_cast<void*>(reinterpret_cast<const void*>(&body))) {}
R operator()(Args... args) const {
return fn(std::forward<Args>(args)..., data);
}
R (*const fn)(void* data, Args... args);
void* const data;
};
} // namespace tcmalloc
#endif // BASE_FUNCTION_REF_H_