[c++] Difference between const reference and normal parameter

The important difference is that when passing by const reference, no new object is created. In the function body, the parameter is effectively an alias for the object passed in.

Because the reference is a const reference the function body cannot directly change the value of that object. This has a similar property to passing by value where the function body also cannot change the value of the object that was passed in, in this case because the parameter is a copy.

There are crucial differences. If the parameter is a const reference, but the object passed it was not in fact const then the value of the object may be changed during the function call itself.

E.g.

int a;

void DoWork(const int &n)
{
    a = n * 2;  // If n was a reference to a, n will have been doubled 

    f();  // Might change the value of whatever n refers to 
}

int main()
{
    DoWork(a);
}

Also if the object passed in was not actually const then the function could (even if it is ill advised) change its value with a cast.

e.g.

void DoWork(const int &n)
{
    const_cast<int&>(n) = 22;
}

This would cause undefined behaviour if the object passed in was actually const.

When the parameter is passed by const reference, extra costs include dereferencing, worse object locality, fewer opportunities for compile optimizing.

When the parameter is passed by value and extra cost is the need to create a parameter copy. Typically this is only of concern when the object type is large.

Examples related to c++

Method Call Chaining; returning a pointer vs a reference? How can I tell if an algorithm is efficient? Difference between opening a file in binary vs text How can compare-and-swap be used for a wait-free mutual exclusion for any shared data structure? Install Qt on Ubuntu #include errors detected in vscode Cannot open include file: 'stdio.h' - Visual Studio Community 2017 - C++ Error How to fix the error "Windows SDK version 8.1" was not found? Visual Studio 2017 errors on standard headers How do I check if a Key is pressed on C++

Examples related to parameters

Stored procedure with default parameters AngularJS ui router passing data between states without URL C#: HttpClient with POST parameters HTTP Request in Swift with POST method In Swift how to call method with parameters on GCD main thread? How to pass parameters to maven build using pom.xml? Default Values to Stored Procedure in Oracle How do you run a .exe with parameters using vba's shell()? How to set table name in dynamic SQL query? How to pass parameters or arguments into a gradle task

Examples related to reference

Method Call Chaining; returning a pointer vs a reference? When to create variables (memory management) Reference to non-static member function must be called Cannot find reference 'xxx' in __init__.py - Python / Pycharm c++ "Incomplete type not allowed" error accessing class reference information (Circular dependency with forward declaration) C++ initial value of reference to non-const must be an lvalue Dependent DLL is not getting copied to the build output folder in Visual Studio How to write to error log file in PHP How to reference Microsoft.Office.Interop.Excel dll? Linker Error C++ "undefined reference "

Examples related to constants

Constants in Kotlin -- what's a recommended way to create them? Why Is `Export Default Const` invalid? Proper use of const for defining functions in JavaScript Declaring static constants in ES6 classes? How can I get the size of an std::vector as an int? invalid use of non-static member function Why does JSHint throw a warning if I am using const? Differences Between vbLf, vbCrLf & vbCr Constants Constant pointer vs Pointer to constant Const in JavaScript: when to use it and is it necessary?