The relationship between C++ function return value types and function signatures

In C, the function return value type is an important part of the function signature. It specifies the data type returned by the function and must match the type actually returned by the function. A function signature contains the function name, a parameter list, and a return type, which is the data type that the function will return, which can be a primitive type, an object type, or void (which means no value is returned). Therefore, a function cannot return a different type than the one specified in the signature, a void function cannot return any value, and both reference types and objects are acceptable as return value types.

The relationship between function return value type and function signature

In C, the return value type of a function is in the function signature Very important part. It specifies what type of data the function will return and must match the type of data the function actually returns.

Function signature

The function signature contains the name of the function, the parameter list and the return value type. It is essentially the function's identifier, which is used by the compiler to identify the function and type-check it.

Example:

int sum(int a, int b); // 函数签名，返回 int 型

Return value type

The return value type is what the function listed in the function signature will return type of data. It can be a primitive data type (such as int, double), an object type, or void (meaning that the function returns no value).

Example:

double calculateArea(double radius); // 返回 double 型
bool isEven(int number); // 返回 bool 型
void printMessage(const string& message); // 不返回任何值（void）

Practical case

Consider the following function that calculates the sum of two integers and returns the result:

int sum(int a, int b) {
  return a + b;
}

• Function signature: int sum(int a, int b)
• Return value type: int

In this example , the return value type int matches the data type actually returned by the function (that is, the sum of two integers). Therefore, the compiler will be able to type-check the function correctly.

Points to note:

• A function cannot return a different type than the type specified in the signature.
• void function cannot return any value (that is, it cannot contain a return statement).
• Reference types (such as pointers and references) can be used as return value types.
• A function can return an object, but in this case the correct type must be used in the function signature.

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