Initialize static variables in C++ class?

2019-01-04 23:38发布

I have noticed that some of my functions in a class are actually not accessing the object, so I made them static. Then the compiler told me that all variables they access must also be static – well, quite understandable so far. I have a bunch of string variables such as

string RE_ANY = "([^\\n]*)";
string RE_ANY_RELUCTANT = "([^\\n]*?)";

and so on in the class. I have then made them all static const because they never change. However, my program only compiles if I move them out of the class: Otherwise, MSVC++2010 complains "Only static constant integral variables may be initialized within a class".

Well that's unfortunate. Is there a workaround? I would like to leave them inside the class they belong to.

9条回答
对你真心纯属浪费
2楼-- · 2019-01-05 00:26

Optionally, move all your constants to .cpp file without declaration in .h file. Use anonymous namespace to make them invisible beyond the cpp module.

// MyClass.cpp

#include "MyClass.h"

// anonymous namespace
namespace
{
    string RE_ANY = "([^\\n]*)";
    string RE_ANY_RELUCTANT = "([^\\n]*?)";
}

// member function (static or not)
bool MyClass::foo()
{
    // logic that uses constants
    return RE_ANY_RELUCTANT.size() > 0;
}
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闹够了就滚
3楼-- · 2019-01-05 00:32

Just to add on top of the other answers. In order to initialize a complex static member, you can do it as follows:

Declare your static member as usual.

// myClass.h
class myClass
{
static complexClass s_complex;
//...
};

Make a small function to initialize your class if it's not trivial to do so. This will be called just the one time the static member is initialized. (Note that the copy constructor of complexClass will be used, so it should be well defined).

//class.cpp    
#include myClass.h
complexClass initFunction()
{
    complexClass c;
    c.add(...);
    c.compute(...);
    c.sort(...);
    // Etc.
    return c;
}

complexClass myClass::s_complex = initFunction();
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Deceive 欺骗
4楼-- · 2019-01-05 00:32

If your goal is to initialize the static variable in your header file (instead of a *.cpp file, which you may want if you are sticking to a "header only" idiom), then you can work around the initialization problem by using a template. Templated static variables can be initialized in a header, without causing multiple symbols to be defined.

See here for an example:

Static member initialization in a class template

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