c++ - Why can't I define a function inside another function?

Question

This is not a lambda function question, I know that I can assign a lambda to a variable.

What's the point of allowing us to declare, but not define a function inside code?

For example:

#include <iostream>

int main()
{
    // This is illegal
    // int one(int bar) { return 13 + bar; }

    // This is legal, but why would I want this?
    int two(int bar);

    // This gets the job done but man it's complicated
    class three{
        int m_iBar;
    public:
        three(int bar):m_iBar(13 + bar){}
        operator int(){return m_iBar;}
    }; 

    std::cout << three(42) << '
';
    return 0;
}

So what I want to know is why would C++ allow two which seems useless, and three which seems far more complicated, but disallow one?

EDIT:

From the answers it seems that there in-code declaration may be able to prevent namespace pollution, what I was hoping to hear though is why the ability to declare functions has been allowed but the ability to define functions has been disallowed.

Answer 1

It is not obvious why one is not allowed; nested functions were proposed a long time ago in N0295 which says:

We discuss the introduction of nested functions into C++. Nested functions are well understood and their introduction requires little effort from either compiler vendors, programmers, or the committee. Nested functions offer significant advantages, [...]

Obviously this proposal was rejected, but since we don't have meeting minutes available online for 1993 we don't have a possible source for the rationale for this rejection.

In fact this proposal is noted in Lambda expressions and closures for C ++ as a possible alternative:

One article [Bre88] and proposal N0295 to the C ++ committee [SH93] suggest adding nested functions to C ++ . Nested functions are similar to lambda expressions, but are defined as statements within a function body, and the resulting closure cannot be used unless that function is active. These proposals also do not include adding a new type for each lambda expression, but instead implementing them more like normal functions, including allowing a special kind of function pointer to refer to them. Both of these proposals predate the addition of templates to C ++ , and so do not mention the use of nested functions in combination with generic algorithms. Also, these proposals have no way to copy local variables into a closure, and so the nested functions they produce are completely unusable outside their enclosing function

Considering we do now have lambdas we are unlikely to see nested functions since, as the paper outlines, they are alternatives for the same problem and nested functions have several limitations relative to lambdas.

As for this part of your question:

// This is legal, but why would I want this?
int two(int bar);

There are cases where this would be a useful way to call the function you want. The draft C++ standard section 3.4.1 [basic.lookup.unqual] gives us one interesting example:

namespace NS {
    class T { };
    void f(T);
    void g(T, int);
}

NS::T parm;
void g(NS::T, float);

int main() {
    f(parm); // OK: calls NS::f
    extern void g(NS::T, float);
    g(parm, 1); // OK: calls g(NS::T, float)
}