Function Signature — What is it?

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Simply put, a Function Signature is a unique identifier for a function in a program. It is the hash of a function’s component which are:

1. Function visibility: This refers to whether the function is public, private or protected. It defines the scope of where the function can be accessed from within the code.
2. Function name: This is the unique name given to a function. It is used to call or reference the function in the program.
3. Function arguments: These are the inputs that a function receives. They can be one or many. They can vary in number and type.
4. Function return types: This refers to the kind of value that the function returns. It could be a single value, an array of values, an object or any data type.
5. Function modifiers: Modifiers are keywords that are added to the definitions of functions to change their behaviour. They can be used to control access to the function, to modify the return type of the function, or to impose other restrictions or allowances on the function.

Let me analyze each of these components.

Function Visibility

Function visibility defines where a function can be accessed from. There are 4 main types of visibility in programming languages.

1. Public Visibility: If a function is public, it can be accessed from anywhere in the code, even outside the class or struct it resides in.
2. Private Visibility: Private functions are only accessible within the class they are defined. (Emphasis on only)
3. Protected Visibility: Protected functions can be accessed within the class they are defined it and by the subclasses of that class.

Other Visibility types exists: We have the Static, Internal, External and they differ in programming languages. Solidity, Java, C++ etc. have these visibility types.

Function Name

The function name is the name of a function. It is a unique identifier for our functions when writing code. A function needs to be unique within the scope it is defined.

• Just to chip this in, a good practice when naming functions, is to name functions based on what they do.

Function Arguments

Function arguments are the values passed into the function. Function arguments can be of any data type, and a function can take multiple arguments. Arguments to a function contribute to a function’s uniqueness as different functions can have different number of arguments with different argument types.

Function Return Types

A Function return type also contribute to a function’s uniqueness. A function’s return type is the value type a function returns once that function has completed execution and it could be of any data type including void if the function returns nothing. The function return type is a part of the function signature because it helps distinguish functions with the same name and same arguments but with a different return type.

Function Modifiers

Function Modifiers can change the behaviour of a function. They can control access, modify the return type, or impose restrictions to a function. Modifiers are a powerful tool for making code more concise, readable, and safe. To understand how function modifiers work, I’ll demonstrate using a solidity code snippet. This is because modifiers are common practice in solidity contracts.

pragma solidity ^0.8.18;

contract Example {
    address public owner;
    constructor() public {
        owner = msg.sender;
    }
    modifier onlyOwner {
        require(msg.sender == owner, "Only the contract owner can call this function");
        _;
    }
    // This function can only be called by the owner of the contract because of the modifier onlyOwner.
    function doSomething() public onlyOwner {
        // logic for doSomething
    }
}

Now, from the code, there is a function modifier called onlyOwner that checks if the sender of the transaction is the owner of the contract. If the owner differs, the function doSomething throws an error and stops execution. If the owner of the contract is the one calling the contract, the function executes successfully. The onlyOwner is a function modifier that controls access to the function doSomething.

Are Function Signatures useful?

You might be wondering, what is the usefulness of function signatures? The answer is very useful. Function Signatures are used in compilers which means they are used in every programming language. For instance.

1. Function signatures are used when executing low-level calls between contracts in solidity.
2. Function signatures are used for call instructions when making function calls in C++ and other low-level languages.
3. Identification of functions in languages that supports function overloading.

So, function signatures are very much useful in programming. Think about this, how will programming languages that supports function overloading like Java language be able to manage overloaded functions.

Summary

Function signatures provide a comprehensive overview of what a function does, how it does it, and where it can be accessed from. They are essential for compiling our code in various programming languages.

To understand the work of function signatures during compilation. You can read my article on C++ compiler — Check the link below.

Other Resources

You can check out these resources for more understanding of the terms and concepts written in this article.

• How the C++ Compiler works

I believe you have learnt something from this article.

Thanks for reading. 🙂