How does C++ Lambda expression perform type inference?

C++ Type inference for lambda expressions allows the compiler to determine the return value type of the lambda: if the lambda has only one return statement, the return value type is the type of the return expression. If a lambda has multiple return statements, the return type is one of these types (the compiler may issue a warning). If there is no return statement, the return value type is void.

Type inference of Lambda expressions in C++

Lambda expression is a powerful syntax feature in C++ that allows Create anonymous functions at runtime. Type inference for lambda expressions is a key feature that enables the compiler to determine the return value type of a lambda expression.

Syntax

Lambda expressions are usually defined using the following syntax:

auto lambda = [capture list] (parameter list) -> return-type {
    // lambda function body
}

where:

• [ capture list] is an optional capture list that specifies external variables that the lambda expression can access.
• (parameter list) is an optional parameter list that specifies the parameters received by the lambda expression.
• -> return-type is an optional return type specifier that specifies the return value type of the lambda expression.

Type inference

If the return value type is not specified, the compiler will try to infer it based on the following rules in lambda expressions:

1. If the lambda expression contains a return statement, the return value type is the type of the return expression.
2. If the lambda expression contains one or more return statements, but they return values ​​of different types, the return value type is any of those types. In this case, the compiler may issue a warning.
3. If the lambda expression does not contain a return statement, the return value type is void.

Practical case

Case 1: Using type inference

The following lambda expression uses type inference and returns The value type is int:

auto lambda = [] (int a, int b) {
    return a + b;
};

Case 2: Specify the return value type

The following lambda expression explicitly specifies the return value type as std::string:

auto lambda = [] (std::string a, std::string b) -> std::string {
    return a + b;
};

Case 3: Inferring multiple return types

The following lambda expression contains two return statements , the return type is inferred to be int or std::string:

auto lambda = [] (int a, std::string b) {
    if (a < 0) {
        return a;
    } else {
        return b;
    }
};

The above is the detailed content of How does C++ Lambda expression perform type inference?. For more information, please follow other related articles on the PHP Chinese website!