[英]Programmatically creating functions in c++
假设我们有一个包含 1000 个 function 副本的代码,格式如下:
function myname_[X](args){
struct somevariable_[X];
//others
}
其中 [X] 是一个序列,例如 0001、0002 等。
我打算通过以编程方式生成 function 和变量名称来使其更清晰。 我们如何在 c++ 中做到这一点?
您可以使用模板生成编译时固定数量的 c 样式函数,每个函数都可以将硬编码参数传递给 C++ 回调。 您可以使用该硬编码参数将 c 样式回调与 object 相关联。
这是一个最小的工作 C++14 示例,它允许将唯一的 C++ std::function
与每个生成的 c 样式回调相关联。 您可以使用 lambda 来初始化std::function
,该 lambda 捕获您在该特定回调中需要的任何 state :
#include <iostream>
#include <algorithm>
#include <functional>
class Callbacks {
static constexpr unsigned count = 1000;
static Callbacks* instance;
using CF = void(); // C-style callback type.
using F = std::function<CF>; // C++ stateful callback type.
F callbacks_[count];
CF* c_style_callbacks_[count];
template<unsigned Index>
static void c_style_callback() {
instance->callbacks_[Index]();
}
template<unsigned... Index>
void make_c_style_callbacks(std::integer_sequence<unsigned, Index...>) {
auto initializer_list = {(c_style_callbacks_[Index] = &c_style_callback<Index>)...};
static_cast<void>(initializer_list);
}
public:
Callbacks() {
make_c_style_callbacks(std::make_integer_sequence<unsigned, count>{});
if(instance)
throw; // One instance only please.
instance = this;
}
Callbacks(Callbacks const&) = delete;
Callbacks& operator=(Callbacks const&) = delete;
~Callbacks() noexcept {
instance = 0;
}
CF* register_callback(F f) noexcept {
// Linear search can be improved upon.
auto condition = [](F const& f) { return !f; };
auto index = std::find_if(std::begin(callbacks_), std::end(callbacks_), condition) - std::begin(callbacks_);
if(index < count) {
callbacks_[index] = std::move(f); // Assumes move-assignement is noexcept.
return c_style_callbacks_[index];
}
return 0;
}
void unregister_callback(CF* cf) noexcept {
// Linear search can be improved upon.
auto index = std::find(std::begin(c_style_callbacks_), std::end(c_style_callbacks_), cf) - std::begin(c_style_callbacks_);
if(index < count)
callbacks_[index] = {};
else
throw; // cf has not been found. Programming error.
}
};
Callbacks* Callbacks::instance = 0;
int main() {
Callbacks c;
unsigned n = 0;
auto p0 = c.register_callback([m = n++]() { std::cout << m << '\n'; });
auto p1 = c.register_callback([m = n++]() { std::cout << m << '\n'; });
auto p2 = c.register_callback([m = n++]() { std::cout << m << '\n'; });
p0(); // Outputs 0.
p1(); // Outputs 1.
p2(); // Outputs 2.
c.unregister_callback(p2);
c.unregister_callback(p1);
c.unregister_callback(p0);
}
该解决方案需要使用一点全局 state,这里是Callbacks::instance
。 An alternative would be to parametrize c_style_callback
with a reference to an object with linkage (internal or external), which means a global, namespace-scoped or class static object.
如果您使用的是 C++11,您将需要使用到达 C++14 但实际上不需要任何 C++14 特定功能的std::integer_sequence
和std::make_integer_sequence
的反向移植。 自 C++14 起,示例[m = n++]
命名捕获也可用,但这仅用于演示。
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