The root cause of encountering the "undefined reference to function" error is that the linker cannot find the implementation of the function. Specific reasons include: ① The function declaration is not defined only; ② The function definition is misspelled or inconsistent; ③ Multiple source files are not correctly linked; ④ The static library/dynamic library is not correctly linked. The solutions are: ? Check whether the function is implemented in a .cpp file; ? Check the consistency of function signatures; ? Ensure that multiple file projects are compiled and linked together; ? Add correct link parameters when calling third-party libraries; ? Add class scope when using class member functions. Just follow the steps to locate and resolve the problem.

An error of "undefined reference to function" is encountered, which means that your C program cannot find the implementation of a certain function during the linking stage. This is not a compilation problem, but an error reported by the linker. Simply put: you declare the function, and you call it, but you don't give the linker an actual function body.

Why does an undefined reference error appear?

This type of error is usually caused by the following reasons:

• Function declaration is only undefined : you wrote the function prototype in the header file or code, but forgot to write the specific implementation of the function.
• Function definition spelling errors : such as inconsistent parameter types, different case of function names, etc.
• Multiple source files are not correctly linked : If you implement the function in another .cpp file, but it is not compiled or linked together at compile time.
• Static/dynamic library is not linked correctly : This error will also occur if the correct linking option is not specified when using third-party libraries.

For example:

 // main.cpp
int add(int a, int b); // Declare int main() {
    int result = add(2, 3); // Call return 0;
}

The above code will not compile errors, but it will prompt undefined reference to 'add(int, int)' when linking, because you did not write the implementation of add function at all.

How to solve this problem?

? Check if the function is really implemented

Make sure that the function you call has a complete function body in a certain .cpp file. For example:

 // add.cpp
int add(int a, int b) {
    return ab;
}

Then when compiling, all relevant .cpp files need to be compiled together, for example:

 g main.cpp add.cpp -o myprogram

If it is not compiled together, the linker cannot find the implementation of add .

? Check whether the function signature is consistent

Sometimes it looks like the same function, but in fact the parameter types or numbers are different, which can also cause link failure. for example:

 // Declaration is int add(int, int)
int add(int a, int b);  

// Implementation is double add(int, int)
double add(int a, int b) {
    return ab;
}

Although the name is the same, the return value and type do not match, which will cause the linker to not find the corresponding function.

? Remember to add scope when using class member functions

If you are calling a member function of the class but forget to add the class name and scope resolution operator :: , an error may also be reported. For example:

 class Math {
public:
    static int square(int x);
};

// Forgot to write Math::square
int square(int x) { // This is a global function, not a return x * x in the class;
}

At this time, calling Math::square(5) will report an undefined reference.

? Third-party libraries need to be linked correctly

If you are calling functions outside the standard library (such as OpenGL, SDL, Boost, etc.), you need to add the corresponding link parameters to the compile command. For example:

 g main.cpp -lGL -lGLU -o myglapp

Here -lGL means linking to the OpenGL library. Without these parameters, the linker will naturally not find those library functions.

Summarize how to check

• [] Check whether there are any missed function implementations.
• [] Check whether the signature of the function declaration and implementation are exactly the same.
• [] Multi-file project confirmation is involved in compilation and linking.
• [] If an external library is used, confirm whether the correct link parameters have been added.
• [] Pay attention to whether the class member function uses the correct qualifiers and scope.

Basically these are the situations. This kind of error may seem scary, but as long as you check it according to the steps, you can generally locate it. The key is to understand that this is a problem at the link stage, not a syntax error.

The above is the detailed content of undefined reference to function error in C. For more information, please follow other related articles on the PHP Chinese website!