CODEWITHSUNDEEP
c++compiler-errorslinker
Peter Zhu
Peter Zhu

Reputation: 1194

'undefined reference' with 'not declared'

this is a simple problem in c compiling and linking. But I want to discuss the principle of compiler and linker.

void f();
int main()
{
    f();
}

this code receive error message "undefined reference to 'f()'"

int main()
{
    f();
}

this code receive error message "f was not declared in this scope" what is the difference? and what 's more, the code

void f()
{

}
int main()
{
    f();
}

runs well.

Upvotes: 0

Views: 1029

Answers (2)

Roger.Kang
Roger.Kang

Reputation: 62

To clarify this question. You should know how the code becomes a executable program. The first step is "Compile". Compile your source code into *.o file which is binary file. The second if "Link". Combine your *.o file into executable file. In your first example, you declare the function f ,but you don't define it. So when the Linker links the *.o file, it can't find function f. In your second example, you use function f without declaration or implementation. So the Compiler report an error. In your third example, the function f implementation is ahead using it. It's right.

Upvotes: 0

user3553031
user3553031

Reputation: 6224

In the first case, you declare a function, but don't define it. It compiles properly, but doesn't link because there is no definition for f. Hence the linker error.

In the second case, you attempt to call an undeclared symbol. The compiler doesn't know what f is, so it issues an error. It's a different problem from a different stage of the compilation process, so the message is different.

In the third case, you have a well-defined program (except that main fails to return a value). f is both declared and defined. The program should compile, link, and execute properly.

Upvotes: 4

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