Returning Lambdas
When you return a lambda from a function, C++ can often deduce the return type automatically using auto. This is convenient and keeps your code concise. However, there are cases where you might want to specify an explicit return type, such as std::function, for greater flexibility.
main.cpp
1234567891011121314151617181920#include <iostream> #include <functional> // Function returning a lambda that multiplies by a captured factor auto make_multiplier(int factor) { // The lambda captures 'factor' by value return [factor](int x) { return x * factor; }; } int main() { // Create a multiplier that multiplies by 5 auto times_five = make_multiplier(5); std::cout << "3 * 5 = " << times_five(3) << std::endl; // Output: 15 // Using std::function as the return type std::function<int(int)> times_ten = make_multiplier(10); std::cout << "4 * 10 = " << times_ten(4) << std::endl; // Output: 40 }
Using std::function as the return type allows you to return any callable object matching the signature, not just a specific lambda. This can be helpful if you want to change the implementation later, or if you need to store different types of callables in a container or pass them around in your code. However, using std::function may introduce some overhead compared to using auto-deduced lambdas directly, so you should choose the approach that best fits your needs.
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Can you give an example of returning a lambda with auto and with std::function?
What are the performance implications of using std::function versus auto?
When should I prefer std::function over auto for returning callables?
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When you return a lambda from a function, C++ can often deduce the return type automatically using auto. This is convenient and keeps your code concise. However, there are cases where you might want to specify an explicit return type, such as std::function, for greater flexibility.
main.cpp
1234567891011121314151617181920#include <iostream> #include <functional> // Function returning a lambda that multiplies by a captured factor auto make_multiplier(int factor) { // The lambda captures 'factor' by value return [factor](int x) { return x * factor; }; } int main() { // Create a multiplier that multiplies by 5 auto times_five = make_multiplier(5); std::cout << "3 * 5 = " << times_five(3) << std::endl; // Output: 15 // Using std::function as the return type std::function<int(int)> times_ten = make_multiplier(10); std::cout << "4 * 10 = " << times_ten(4) << std::endl; // Output: 40 }
Using std::function as the return type allows you to return any callable object matching the signature, not just a specific lambda. This can be helpful if you want to change the implementation later, or if you need to store different types of callables in a container or pass them around in your code. However, using std::function may introduce some overhead compared to using auto-deduced lambdas directly, so you should choose the approach that best fits your needs.
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