C++ – Constant strings address

c++constantsstring-comparison

I have several identical string constants in my program:

const char* Ok()
{
  return "Ok";  
}

int main()
{
  const char* ok = "Ok";
}

Is there guarantee that they are have the same address, i.e. could I write the following code? I heard that GNU C++ optimize strings so they have the same address, could I use that feature in my programs?

int main()
{
  const char* ok = "Ok";
  if ( ok == Ok() ) // is it ok?
  ;
}

Best Solution

There's certainly no guarantee, but it is a common (I think) optimization.

The C++ standard says (2.13.4/2 "String literals):

Whether all string literals are distinct (that is, are stored in nonoverlapping objects) is implementation-defined.

To be clear, you shouldn't write code that assumes this optimization will take place - as Chris Lutz says, C++ code that relies on this is code that's waiting to be broken.

Related Solutions

C++ – Static constant string (class member)

You have to define your static member outside the class definition and provide the initializer there.

First

// In a header file (if it is in a header file in your case)
class A {   
private:      
  static const string RECTANGLE;
};

and then

// In one of the implementation files
const string A::RECTANGLE = "rectangle";

The syntax you were originally trying to use (initializer inside class definition) is only allowed with integral and enum types.

Starting from C++17 you have another option, which is quite similar to your original declaration: inline variables

// In a header file (if it is in a header file in your case)
class A {   
private:      
  inline static const string RECTANGLE = "rectangle";
};

No additional definition is needed.

Starting from C++20 instead of const you can declare it constexpr in this variant. Explicit inline would no longer be necessary, since constexpr implies inline.

Python – How to create a constant in Python

No there is not. You cannot declare a variable or value as constant in Python. Just don't change it.

If you are in a class, the equivalent would be:

class Foo(object):
    CONST_NAME = "Name"

if not, it is just

CONST_NAME = "Name"

But you might want to have a look at the code snippet Constants in Python by Alex Martelli.

As of Python 3.8, there's a typing.Final variable annotation that will tell static type checkers (like mypy) that your variable shouldn't be reassigned. This is the closest equivalent to Java's final. However, it does not actually prevent reassignment:

from typing import Final

a: Final = 1

# Executes fine, but mypy will report an error if you run mypy on this:
a = 2

Best Solution

Related Solutions

C++ – Static constant string (class member)

Python – How to create a constant in Python

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