In C++, lambda expressions are functions that can be defined inline and don't require a separate function declaration. Lambda expressions can have parameters and return types, which makes them very flexible and powerful. Here is an overview of how to define parameters and return types in C++ lambda expressions.

Defining Lambda Parameters:

Lambda parameters are defined inside parentheses after the lambda expression's opening bracket. You can define zero or more parameters, separated by commas. The syntax for defining lambda parameters is as follows:

[ ](parameters) { /* lambda body */ }

For example, you can define a lambda expression that takes two integer parameters and prints their sum:

auto sum = [](int x, int y) { std::cout << x + y << std::endl; };
sum(5, 10); // prints 15

You can also define lambda expressions with no parameters, as follows:

auto print_hello = [] { std::cout << "Hello, world!" << std::endl; };
print_hello(); // prints "Hello, world!"

Defining Lambda Return Types:

In C++, lambda expressions can return a value. The return type is specified after the lambda expression's parameter list and before the opening bracket. If the lambda expression does not return a value, you can use void as the return type. The syntax for defining lambda return types is as follows:

[ ](parameters) -> return_type { /* lambda body */ }

For example, you can define a lambda expression that takes two integer parameters and returns their sum:

auto sum = [](int x, int y) -> int { return x + y; };
std::cout << sum(5, 10) << std::endl; // prints 15

You can also define lambda expressions that don't return a value, like this:

auto print_hello = []() -> void { std::cout << "Hello, world!" << std::endl; };
print_hello(); // prints "Hello, world!"

If the return type of the lambda expression can be inferred from its body, you don't need to specify the return type explicitly. In this case, you can omit the return type altogether and let the compiler deduce it. For example:

auto sum = [](int x, int y) { return x + y; };
std::cout << sum(5, 10) << std::endl; // prints 15

In this example, the return type of the lambda expression is deduced to be int because the lambda expression returns the result of adding x and y.