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The All-in-One Guide to C++20

Template Parameter for Lambdas

Discover different types of lambdas in C++.

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With C++20, lambda expressions support template parameters, and hence concepts can be default-constructed and support copy assignment when they have no state. Additionally, lambda expressions can be used in unevaluated contexts. With C++20, they detect when you implicitly copy the this pointer. This means a significant cause of undefined behaviorAll bets are open. Your program can produce the correct result, the wrong result, crashes during run-time, or may not even compile. That behavior might change when porting to a new platform, upgrading to a new compiler or as a result of an unrelated code change. with lambdas is gone.

Let’s start with template parameters for lambdas.

The subtle differences between different lambdas

Admittedly, the differences between typed lambdas (C++11), generic lambdas (C++14), and template lambdas (template parameter for lambdas) in C++20 are subtle.

C++
#include <iostream>
#include <string>
#include <vector>
auto sumInt = [](int fir, int sec) { return fir + sec; };
auto sumGen = [](auto fir, auto sec) { return fir + sec; };
auto sumDec = [](auto fir, decltype(fir) sec) { return fir + sec; };
auto sumTem = []<typename T>(T fir, T sec) { return fir + sec; };
int main() {
    std::cout << '\n'; 
    std::cout << "sumInt(2000, 11): " << sumInt(2000, 11) << '\n';
    std::cout << "sumGen(2000, 11): " << sumGen(2000, 11) << '\n';
    std::cout << "sumDec(2000, 11): " << sumDec(2000, 11) << '\n';
    std::cout << "sumTem(2000, 11): " << sumTem(2000, 11) << '\n';
    std::cout << '\n';
    std::string hello = "Hello ";
    std::string world = "world";
    // std::cout << "sumInt(hello, world): " << sumInt(hello, world) << '\n';
    std::cout << "sumGen(hello, world): " << sumGen(hello, world) << '\n';
    std::cout << "sumDec(hello, world): " << sumDec(hello, world) << '\n';
    std::cout << "sumTem(hello, world): " << sumTem(hello, world) << '\n';
    std::cout << '\n';
    std::cout << "sumInt(true, 2010): " << sumInt(true, 2010) << '\n';
    std::cout << "sumGen(true, 2010): " << sumGen(true, 2010) << '\n';
    std::cout << "sumDec(true, 2010): " << sumDec(true, 2010) << '\n';
    // std::cout << "sumTem(true, 2010): " << sumTem(true, 2010) << '\n';
    std::cout << '\n';
}

Before discussing the output, I want to compare the four lambdas:

  • sumInt

    C++ version Type of lambda Types of parameters
    C+11 Typed lambda Accepts only types
    convertible to int
...