第十三號艦隊
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C++ lambda interact with C's Callback function

Posted on Edited on In ,

從最簡單的情況說起

煤捕捉任何狀態的lambda可以直接轉化成普通的C function型態

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typedef void(*callback)();
void testFunc(callback cb)
{
	cb();
}
testFunc([] {
	cout << "Lambda function" << endl;
});

帶狀態的情形

通常callback還會帶一個void *參數，可以讓你上下其手

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typedef void(*callback)(void *);
void testFunc(callback cb, void *user_data)
{
	cb(user_data);
}
	int v = 123;
	testFunc([](void *user_data) {
		int *v = static_cast<int *>(user_data);
		cout << "Lambda function " << *v<< endl;
	}, &v);

當需要更多上下文時，需要自行定義structure

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// local variables
int x = 0;
float y = 1;

// locally defined uncopyable, unmovable type
struct MtEverest
{
	MtEverest() = default;
	MtEverest(const MtEverest& that) = delete; // no copy
	MtEverest(const MtEverest&& that) = delete; // no move
} mt_everest;

// create "user-data" payload  
auto payload = std::tie(x, y, mt_everest);
testFunc([](void *user_data) {
	auto& payload_tup = *reinterpret_cast<decltype(payload)*>(user_data);
	auto& xx = std::get<0>(payload_tup);
	auto& yy = std::get<1>(payload_tup);
	auto& me = std::get<2>(payload_tup);
}, &payload);

更高明的解法

當C API沒辦法有void *的指標讓你上下其手，或是packing/unpacking的程式碼讓人不耐
可以考慮以下這個解法

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template <typename Lambda>
static auto lambdacb(Lambda &&l)
{
	thread_local auto* p = &l; // initial assignment, allows using auto
  p = &l;
	return []() { return (*p)(); };
}

typedef void(*callback)();
void testFunc(callback cb)
{
	cb();
}
// local variables
int x = 0;
float y = 1;

testFunc(lambdacb([&x, &y] {
	cout << x << " " << y << endl;
}));

Reference

— Lambda Magic
— Lambda Callbacks
— Technical Debt