Too much declaration and initialization in header file C++

Question

I've a big file of Constants.h file where about 200 variables(mostly arrays) are declared and initialised. I'm using namespace.

METHOD 1:

//Constants.h

#ifndef CONSTANTS_H_
#define CONSTANTS_H_

namespace LibConstants
{
    const int a = 12;
    const std::string theme[2]; = {"themeA", "themeB"};
    const int arr[2] = {1, 2};
    // and around 200 more declarations with initialization
}
#endif

This .h file is #include in almost every .cpp file but each time only very minimal variables are being used like LibConstants::theme[0].

My ways works fine but doesn't it allocate memory unnecessarily? Shall I follow the approach, to only define variables in .h file and initialize in .cpp?

Like in below code: METHOD 2:

//Constants.h

#ifndef CONSTANTS_H_
#define CONSTANTS_H_

namespace LibConstants {

    std::string getMyTheme(int arg);
    std::string getMyThemeName(int arg);

    const int a;
    const std::string theme[2];
    const int arr[2];
    // and around 200 more declarations with initialisation
};
#endif

Initialising in cpp file

//Constants.cpp

#include LibConstants.h
using namespace LibConstants {
     std::string getMyTheme(int arg) {
         theme[2] = {"themeA", "themeB"};
         return theme[arg];
     }
     std::string getMyThemeName(int arg) {
         ...
     }
}

Using it like

//HelloWorld.cpp

#include Constants.h
int main() {
     //METHOD 1:
     std::string a = LibConstants::theme[0]; // arg=0 is dynamic and is being read from localStorage in my case. 
     //METHOD 2:
     std::string a = LibConstants::getMyTheme(0); //Here also arg=0 is dynamic.
     ...
}

Here, unnecessary allocation of memory for unneeded variables won't happen except for arrays which are declared as const std::string st[2]; in the header file.

Here "arg=0" is run time involvement. Does it matter if some variable is not dependent on run-time but only compile time in which case it will simply replace the value of placeholder in corresponding .cpp file?

Please correct me wherever I am wrong.

Answer 1

Take the std::string example. Consider,

namespace Constants {
const std::string str = "A not very short string";
}

str is a class type, it has a constructor, and it has to allocate memory to store its contents (unless short-string optimization is used in its implementation. Even in that case, it only applies to, well, "short string"). It is declared a namespace scope variable. So the program has to construct this variable at launch. Every program that has this variable declared, will need to allocate memory, and initialize its contents. That is an overhead you probably don't want, depending on how much that will effect your performance.

On the other hand, if you have,

const std::string &str()
{
     const static std::string s = "A not very short string";

     return s;
}

Now, you have a static local variable. It will be initialized at the first entry of the function. And it will be initialized only once. If str is never called, it will not be initialized at all.

However, note that the string literal "A not very short string" is still going to occupy some memory. Where and how it is store is implementation defined. Usually in the data segments, and the impact is usually minimal.

In contrast to class types. It is preferable to define fundamental types, especially integral types in the header.

For example,

namespace Constants {
constexpr int x = 10;
}

An optimizing compiler will most likely not to store the variable x and its contents 10 at all. Instead, wherever x is used, it will be replaced by 10, and in some cases, coded into the instruction op-code (so called immediate operands). Of course, this is again an implementation detail. And such optimization cannot be relied on with all compilers. And if you take x's address or otherwise ODR used, compiler will be compelled to make room for this variable anyway. But the bottomline is that, it will be very unlikely that you will be worse off by declaring this constants in header than defining it in an external source file.