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c++constants

What does "const" mean for variables, function parameters, and member functions?

When reading tutorials and code written in C++, I often stumble over the const keyword.

I see that it is used like the following:

const int x = 5;

I know that this means that x is a constant variable and probably stored in read-only memory.

But what are

void myfunc( const char x );

and

int myfunc( ) const;

?

Solution 
  • void myfunc(const char x);

    This means that the parameter x is a char whose value cannot be changed inside the function. For example:

    void myfunc(const char x)
{
  char y = x;  // OK
  x = y;       // failure - x is `const`
}

    For the last one:

    int myfunc() const;

    This is illegal unless it's inside a class declaration - const member functions prevent modification of any class member - const nonmember functions cannot be used. in this case the definition would be something like:

    int myclass::myfunc() const
{
  // do stuff that leaves members unchanged
}

    If you have specific class members that need to be modifiable in const member functions, you can declare them mutable. An example would be a member lock_guard that makes the class's const and non-const member functions threadsafe, but must change during its own internal operation.