In object-oriented programming, especially in languages like C++, developers often encounter the error “Modifiers Not Allowed on Nonmember Functions.” This issue arises when attempting to use class-specific modifiers (such as const, static, or virtual) on functions that do not belong to a class.
Understanding why this error occurs and how to fix it is crucial for writing clean and efficient C++ code. In this topic, we will explain the causes of this error, explore different function types in C++, and provide solutions with best practices.
Understanding Nonmember Functions
A nonmember function is a function that exists outside of a class. It is not associated with any specific class and operates independently.
Example of a Nonmember Function
#include <iostream>using namespace std;void greet() {cout << "Hello, World!" << endl;}int main() {greet(); // Calling the functionreturn 0;}Here, greet() is a nonmember function because it is not defined inside a class.
Why Does the “Modifiers Not Allowed on Nonmember Functions” Error Occur?
In C++, some function modifiers are specifically meant for member functions (functions inside a class). When these modifiers are applied to nonmember functions, a compilation error occurs.
Common Modifiers That Cause This Error
constModifierstaticModifiervirtualModifier
Each of these modifiers serves a specific role in class-based functions but cannot be applied to functions outside a class.
Common Causes of the Error
1. Using const on a Nonmember Function
In C++, the const modifier is used to prevent modification of member variables inside a class. When applied incorrectly to a nonmember function, it causes an error.
Incorrect Code:
#include <iostream>using namespace std;const void showMessage() { // ❌ Error: 'const' cannot be used on a nonmember functioncout << "Hello, World!" << endl;}int main() {showMessage();return 0;}Error Message:
error: 'const' qualifier is not allowed on a function definition outside of a classCorrect Code:
Simply remove const:
void showMessage() {cout << "Hello, World!" << endl;}2. Using static on a Nonmember Function
In C++, static functions inside a class belong to the class itself rather than an instance. However, static at the global scope has a different meaning-it limits the function’s visibility to the current file.
Incorrect Code:
#include <iostream>using namespace std;static void display() { // ❌ Error in some casescout << "This is a static function." << endl;}int main() {display();return 0;}Why This Can Cause an Error:
- In C++ classes,
staticmeans “this function does not depend on an instance.” - In global scope,
staticmeans “this function is limited to this file.” - Using
staticincorrectly inside a class declaration will lead to errors.
Correct Code:
If static is needed, declare it inside a class:
#include <iostream>using namespace std;class Example {public:static void display() { // ✅ Correct usagecout << "This is a static member function." << endl;}};int main() {Example::display();return 0;}3. Using virtual on a Nonmember Function
In C++ polymorphism, the virtual keyword is used to allow derived classes to override functions dynamically. Since nonmember functions are not part of a class, virtual cannot be used on them.
Incorrect Code:
#include <iostream>using namespace std;virtual void logMessage() { // ❌ Error: 'virtual' is only for member functionscout << "Logging message..." << endl;}int main() {logMessage();return 0;}Error Message:
error: 'virtual' outside class declarationCorrect Code:
To use virtual, the function must be a member of a class:
#include <iostream>using namespace std;class Logger {public:virtual void logMessage() { // ✅ Correct usagecout << "Logging message..." << endl;}};class FileLogger : public Logger {public:void logMessage() override { // Overriding the functioncout << "Logging to a file..." << endl;}};int main() {FileLogger logger;logger.logMessage(); // Calls the overridden functionreturn 0;}How to Fix the Error
If you encounter the “Modifiers Not Allowed on Nonmember Functions” error, follow these steps:
✅ Check if the function is inside a class.
✅ Remove incorrect modifiers (const, static, virtual) if the function is not a class member.
✅ Use the correct syntax when defining class-based functions.
Best Practices for Function Modifiers in C++
1. Use const Only for Member Functions
If a function does not modify class data, mark it as const:
class Example {public:void show() const { // ✅ Correct usage of 'const'cout << "This is a constant function." << endl;}};2. Use static Correctly
If a function should belong to the class rather than an instance, mark it as static:
class Math {public:static int add(int a, int b) { // ✅ Correct static functionreturn a + b;}};3. Use virtual for Polymorphism
Use virtual only inside a class to enable function overriding:
class Base {public:virtual void show() { // ✅ Correct use of 'virtual'cout << "Base class function" << endl;}};The error “Modifiers Not Allowed on Nonmember Functions” occurs when using class-specific function modifiers (const, static, virtual) on nonmember functions.
Key Takeaways:
✔ Nonmember functions do not belong to a class.
✔ const is only for member functions that do not modify data.
✔ static should be used inside classes, not incorrectly on nonmember functions.
✔ virtual applies only to class member functions for polymorphism.
By following these best practices, you can avoid syntax errors and write clean, maintainable C++ code.