C++14中lambda表达式新增加的features的使用

news2025/2/26 22:14:39

lambda表达式是在C++11中引入的，它们可以嵌套在其它函数甚至函数调用语句中，C++11中lambda表达式的使用参考：https://blog.csdn.net/fengbingchun/article/details/52653313

lambda表达式语法如下：除capture和body是必须的，其它均是可选的

[capture] (params) mutable exception attribute -> return-type { body }

这里介绍下C++14中对lambda表达式新增加的features:

1.generic lambda：lambda表达式与auto关键字组合，将auto用作参数类型，以下为测试代码：

namespace {

//void print_elements(auto& C) // windows need to c++20
template<typename T>
void print_elements(const T& C)
{
	for (const auto& e : C)
		std::cout << e << " ";
	std::cout << "\n";
}

}

int test_lambda_14_1()
{
	// reference: https://www.geeksforgeeks.org/generalized-lambda-expressions-c14/
	/* Under the hood, the C++ implementation uses the closure type’s operator() to overload a template function
	struct sum {
		template<typename T1, typename T2>
		auto operator()(T1 a, T2 b) const { return (a + b); }
	};
	*/
	auto sum = [](auto a, auto b) {
		return (a + b);
	};

	std::cout << "int sum:" << sum(10, 20) << "\n";
	std::cout << "float sum:" << sum(1.2f, 2.3f) << "\n";
	std::cout << "float sum:" << sum(10, 1.5f) << "\n";
	std::cout << "string sum:" << sum(std::string("csdn addr:"), std::string("https://blog.csdn.net/fengbingchun")) << "\n";


	auto greater = [](auto a, auto b) -> bool {
		return (a > b);
	};

	std::vector<int> vi = { 1, 4, 2, 6 };
	std::vector<float> vf = { 4.62f, 161.3f, 62.26f, 13.4f };
	std::vector<std::string> vs = { "Tom", "Harry", "Ram", "Shyam" };

	std::sort(vi.begin(), vi.end(), greater);
	std::sort(vf.begin(), vf.end(), greater);
	std::sort(vs.begin(), vs.end(), greater);

	print_elements(vi);
	print_elements(vf);
	print_elements(vs);


	std::vector<std::vector<int>> v = { {7, 8}, {1, 2}, {3, 7}, {4, 5} };
	std::sort(v.begin(), v.end(), [](std::vector<int>& a, std::vector<int>& b) {
		return (a[1] < b[1]);
	});

	for (int i = 0; i < v.size(); ++i) {
		for (int j = 0; j < v[0].size(); ++j) {
			std::cout << v[i][j] << " ";
		}
		std::cout << "\n";
	}

	return 0;
}

执行结果如下图所示：

2.capture initializers：允许创建使用任意表达式初始化captures。初始化表达式在创建lambda时计算，而不是在调用时，以下为测试代码：

namespace { int factory(int i) { return i * 10; } }

int test_lambda_14_2()
{
	// reference: https://github.com/AnthonyCalandra/modern-cpp-features#lambda-capture-initializers
	auto f = [x = factory(2)] { return x; };
	std::cout << "f:" << f() << "\n";


	auto generator = [x = 0]() mutable {
		// this would not compile without 'mutable' as we are modifying x on each call
		return x++;
	};

	auto a = generator();
	auto b = generator();
	auto c = generator();
	std::cout << "a:" << a << ",b:" << b << ",c:" << c << "\n";


	auto p = std::make_unique<int>(1);
	//auto task1 = [=] { *p = 5; }; // ERROR: std::unique_ptr cannot be copied
	auto task2 = [p = std::move(p)] { *p = 5; return *p; }; // OK: p is move-constructed into the closure object
	// the original p is empty after task2 is created
	if (!p)
		std::cout << "p is empty" << "\n";
	std::cout << "task2:" << task2() << "\n";


	auto x = 2;
	auto f2 = [&r = x, x = x * 10] {
		++r;
		return r + x;
	};
	std::cout << "f2:" << f2() << ",x:" << x << "\n";

	return 0;
}

执行结果如下图所示：