Is there some ninja trick to make a variable constant after its declaration?

I know the answer is 99.99% no, but I figured it was worth a try, you never know.

void SomeFunction(int a)
{
    // Here some processing happens on a, for example:
    a *= 50;
    a %= 10;
    if(example())
       a = 0;
    // From this point on I want to make "a" const; I don't want to allow
    // any code past this comment to modify it in any way.
}

I can do something somewhat similar with const int b = a;, but it's not really the same and it creates a lot of confusion. A C++0x-only solution is acceptable.

EDIT: another less abstracted example, the one that made me ask this question:

void OpenFile(string path)
{
    boost::to_lower(path);
    // I want path to be constant now
    ifstream ...
}

EDIT: another concrete example: Recapture const-ness on variables in a parallel section.

c++ c++11 const-correctness

Edit Report

1 Answer

Why not refactor your code in to two separate functions. One that returns a modified a and another that works on this value (without ever changing it).

You could possibly wrap your object too around a holder class object and work with this holder.

template <class T>
struct Constify {
    Constify(T val) : v_( val ) {}
    const T& get() const  { return v_; }
};

void SomeFuncion() {
    Constify ci( Compute() ); // Compute returns `a`
    // process with ci
}

Your example has an easy fix: Refactoring.

// expect a lowercase path or use a case insensitive comparator for basic_string
void OpenFile(string const& path)  
{        
    // I want path to be constant now
    ifstream ...
}

OpenFile( boost::to_lower(path) ); // temporaries can bind to const&

answered 2010-09-08T15:30:49.170

Is there some ninja trick to make a variable constant after its declaration?

1 Answer

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