Why do functions need to be declared before they are used?

Question

When reading through some answers to this question, I started wondering why the compiler actually does need to know about a function when it first encounters it. Wouldn't it be simple to just add an extra pass when parsing a compilation unit that collects all symbols declared within, so that the order in which they are declared and used does not matter anymore?

One could argue, that declaring functions before they are used certainly is good style, but I am wondering, is there are any other reason why this is mandatory in C++?

Edit - An example to illustrate: Suppose you have to functions that are defined inline in a header file. These two function call each other (maybe a recursive tree traversal, where odd and even layers of the tree are handled differently). The only way to resolve this would be to make a forward declaration of one of the functions before the other.

A more common example (though with classes, not functions) is the case of classes with private constructors and factories. The factory needs to know the class in order to create instances of it, and the class needs to know the factory for the friend declaration.

If this is requirement is from the olden days, why was it not removed at some point? It would not break existing code, would it?

Answer 1

How do you propose to resolve undeclared identifiers that are defined in a different translation unit?

C++ has no module concept, but has separate translation as an inheritance from C. A C++ compiler will compile each translation unit by itself, not knowing anything about other translation units at all. (Except that export broke this, which is probably why it, sadly, never took off.)
Header files, which is where you usually put declarations of identifiers which are defined in other translation units, actually are just a very clumsy way of slipping the same declarations into different translation units. They will not make the compiler aware of there being other translation units with identifiers being defined in them.

Edit re your additional examples:
With all the textual inclusion instead of a proper module concept, compilation already takes agonizingly long for C++, so requiring another compilation pass (where compilation already is split into several passes, not all of which can be optimized and merged, IIRC) would worsen an already bad problem. And changing this would probably alter overload resolution in some scenarios and thus break existing code.

Note that C++ does require an additional pass for parsing class definitions, since member functions defined inline in the class definition are parsed as if they were defined right behind the class definition. However, this was decided when C with Classes was thought up, so there was no existing code base to break.

Answer 2

Historically C89 let you do this. The first time the compiler saw a use of a function and it didn't have a predefined prototype, it "created" a prototype that matched the use of the function.

When C++ decided to add strict typechecking to the compiler, it was decided that prototypes were now required. Also, C++ inherited the single-pass compilation from C, so it couldn't add a second pass to resolved all symbols.

Answer 3

Because C and C++ are old languages. Early compilers didn't have a lot of memory, so these languages were designed so a compiler can just read the file from top to bottom, without having to consider the file as a whole.

Answer 4

I think of two reasons:

• It makes the parsing easy. No extra pass needed.
• It also defines scope; symbols/names are available only after its declaration. Means, if I declare a global variable int g_count;, the later code after this line can use it, but not the code before the line! Same argument for global functions.

As an example, consider this code:

void g(double)
{
    cout << "void g(double)" << endl;
}
void f()
{
    g(int());//this calls g(double) - because that is what is visible here
}
void g(int)
{
    cout << "void g(int)" << endl;
}
int main()
{
    f();
    g(int());//calls g(int) - because that is what is the best match!
}

Output:

void g(double)
void g(int)

See the output at ideone : http://www.ideone.com/EsK4A

Answer 5

The main reason will be to make the compilation process as efficient as possible. If you add an extra pass you're adding both time and storage. Remember that C++ was developed back before the time of Quad Core Processors :)

Answer 6

The C programming language was designed so that the compiler could be implemented as a one-pass compiler. In such a compiler, each compilation phase is only executed once. In such a compiler you cannot referrer to an entity that is defined later in the source file.

Moreover, in C, the compiler only interpret a single compilation unit (generally a .c file and all the included .h files) at a time. So you needed a mechanism to referrer to a function defined in another compilation unit.

The decision to allow one-pass compiler and to be able to split a project in small compilation unit was taken because at the time the memory and the processing power available was really tight. And allowing forward-declaration could easily solve the issue with a single feature.

The C++ language was derived from C and inherited the feature from it (as it wanted to be as compatible with C as possible to ease the transition).

Answer 7

One of the biggest reasons why this was made mandatory even in C99 (compared to C89, where you could have implicitly-declared functions) is that implicit declarations are very error-prone. Consider the following code:

First file:

#include <stdio.h>
void doSomething(double x, double y)
{
    printf("%g %g\n",x,y);
}

Second file:

int main()
{
    doSomething(12345,67890);
    return 0;
}

This program is a syntactically valid* C89 program. You can compile it with GCC using this command (assuming the source files are named test.c and test0.c):

gcc -std=c89 -pedantic-errors test.c test0.c -o test

Why does it print something strange (at least on linux-x86 and linux-amd64)? Can you spot the problem in the code at a glance? Now try replacing c89 with c99 in the command line — and you'll be immediately notified about your mistake by the compiler.

Same with C++. But in C++ there're actually other important reasons why function declarations are needed, they are discussed in other answers.

_{* But has undefined behavior}

Answer 8

I guess because C is quite old and at the time C was designed efficient compilation was a problem because CPUs were much slower.

Answer 9

Since C++ is a static language, the compiler needs to check if values' type is compatible with the type expected in the function's parameters. Of course, if you don't know the function signature, you can't do this kind of checks, thus defying the purpose of a static compiler. But, since you have a silver badge in C++, I think you already know this.

The C++ language specs were made right because the designer didn't want to force a multi-pass compiler, when hardware was not as fast as the one available today. In the end, I think that, if C++ was designed today, this imposition would go away but then, we would have another language :-).

Answer 10

Still, you can have a use of a function before it is declared sometimes (to be strict in the wording: "before" is about the order in which the program source is read) -- inside a class!:

class A {
public:
  static void foo(void) {
    bar();
  }
private:
  static void bar(void) {
    return;
  }
};

int main() {
  A::foo();
  return 0;
}

(Changing the class to a namespace doesn't work, per my tests.)

That's probably because the compiler actually puts the member-function definitions from inside the class right after the class declaration, as someone has pointed it out here in the answers.

The same approach could be applied to the whole source file: first, drop everything but declaration, then handle everything postponed. (Either a two-pass compiler, or large enough memory to hold the postponed source code.)

Haha! So, they thought a whole source file would be too large to hold in the memory, but a single class with function definitions wouldn't: they can allow for a whole class to sit in the memory and wait until the declaration is filtered out (or do a 2nd pass for the source code of classes)!

Answer 11

I remember with Unix and Linux, you have Global and Local. Within your own environment local works for functions, but does not work for Global(system). You must declare the function Global.