The difference is subtle. When you declare something, you're simply stating its name and type. When you define something, you're actually creating an object in memory. A declaration doesn't result in memory being allocated, but a definition does.

The reason they often look the same is that a definition is always a declaration (but a declaration isn't always a definition).

>Please correct me if I'm wrong (which happens a lot)
Well, aside from only taking functions into consideration (declarations and definitions are used for variables too), if you swap your use of declare and define, you would be correct.

A function declaration, also called a prototype, doesn't include the function body:

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void f(); // Declaration

void f(); // Declaration

A function definition does include the function body:

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void f()
{
  // Shtuff
}

void f() { // Shtuff }

Now, because a definition is always a declaration, you can do this:

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// No explicit declaration
 
void f()
{
  // Shtuff
}
 
int main()
{
  f();
}

// No explicit declaration void f() { // Shtuff } int main() { f(); }