How to implement static classes in c++?

Question

I was wanting to implement a simple static class that calculates the pow value of an integer in c++ while practicing. So my codes here:

#pragma once
#ifndef MATH_H
#define MATH_H
static class Math
{
    public:
        static int pow(int,int);
};


#endif /* MATH_H */

And the implemetation of pow function:

#include "Math.h"

int Math::pow(int base, int exp){
    if(exp==1)
        return base;
    else if(exp%2==0)
        return pow(base,exp/2)*pow(base,exp/2);
    else
        return pow(base,exp/2)*pow(base,exp/2)*base;
}

But the cygwin compiler is throwing compilation error:

In file included from Math.cpp:16:0:
Math.h:16:1: error: a storage class can only be specified for objects and functions
 static class Math
 ^~~~~~

Please help me solve my problem.

C++ doesn't have static classes. But it looks like all you need is a function. — juanchopanza
– juanchopanza, Commented Oct 15, 2019 at 18:12
BTW, there is a std::pow function. If you want to use your function, prefix with Math::, e.g. Math::pow. — Thomas Matthews
– Thomas Matthews, Commented Oct 15, 2019 at 18:23

cdhowie · Accepted Answer · 2019-10-15 18:21:09Z

7

C++ does not have "static classes" as they formally exist in other languages. You can, however, delete the default constructor (C++ >= 11) or make it private and unimplemented (C++ < 11) which has the same effect of making the type not constructible:

// C++ >= 11
class Math
{
public:
    Math() = delete;
};

// C++ < 11
class Math
{
private:
    Math(); // leave unimplemented
};

With this code, the line Math m; will fail:

(C++ >= 11) error: use of deleted function 'Math::Math()'

(C++ < 11) error: 'Math::Math()' is private within this context

However, "static classes" are mostly an anti-pattern in C++ and free functions should be preferred. (There are some cases where static classes can be useful, particularly when doing template metaprogramming. If you're not doing that then you almost certainly don't need a static class.)

Consider instead declaring the functions in a namespace:

namespace math {
    int pow(int, int);
}

edited Oct 15, 2019 at 18:21

answered Oct 15, 2019 at 18:13

cdhowie

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12 Comments

Saswata Mishra Over a year ago

But Microsoft is presenting a course on the Edx platform and they are writing such static class containing c++ codes in Visual studio. :(

cdhowie Over a year ago

@SaswataMishra Are you sure that they aren't using the C++/CLI language? Do you see ref class anywhere?

Saswata Mishra Over a year ago

Still haven't seen ref class. But yea, they are using c++.

cdhowie Over a year ago

@SaswataMishra It's entirely possible that the example code you were shown is incorrect. It's also possible that Microsoft's C++ compiler accepts non-standard things that no other compiler does... it wouldn't be the first time. Regardless, static class is invalid C++ according to the standard. Whether Microsoft's compiler accepts it is their problem.

cdhowie Over a year ago

I also seriously question the quality of any C++ course that even teaches about classes that aren't constructable. This is not something you should use in C++ unless you have a very good reason which is almost always template metaprogramming. Using an unconstructable class as a container for functions is wrong in C++. Languages like C# have this feature because they doesn't support free functions at all.

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