Does function definition declare function?

Question

Say a function is defined before it is called:

int test(int i) {
    /* do something */
}

Does defining a function declare it?

Answer 1

int func();

This is a function declaration; it does not provide the body of the function, but it does tell the compiler that it can use this function and expect that it will be defined somewhere.

int func()
{
    return 2;
}

This is a function definition. Defining something means providing all of the necessary information to create that thing in its entirety. Defining a function means providing a function body; defining a class means giving all of the methods of the class and the fields. Once something is defined, that also counts as declaring it; so you can often both declare and define a function, class or variable at the same time. But you don't have to.

So to answer your question: yes

from: http://www.cprogramming.com/declare_vs_define.html

Answer 2

The function definition contains a function declaration and the body of a function. The body is a block of statements that perform the work of the function. The identifiers declared in this example allocate storage; they are both declarations and definitions.

Check out here for more info.

Answer 3

Directly from Wikipedia:

A function prototype or function interface in C, Perl, PHP or C++ is a declaration of a function that omits the function body but does specify the function's return type, name, arity and argument types. While a function definition specifies what a function does, a function prototype can be thought of as specifying its interface.

This function is usable:

int test(int i) {
    /* do something */
}

int main() {
    int k = test(5);
    return 0;
}

However, the order of usage matters when a function is automatically defined, rather than declared.

This code doesn't work, because when test() is invoked, it isn't declared or defined.

int main() {
    int k = test(5);
    return 0
}

int test(int i) {
    /* do something */
}

By declaring all of your functions before hand, you don't need to worry about the order in which they are defined, or invoked (as long as all invocation happens after the declaration). Also it is a good habit and can help you when you've dealing with a large code base with multiple C and header files. The following code compiles because of the declaration:

int test(int);    

int main() {
    int k = test(5);
    return 0
}

int test(int i) {
    /* do something */
}