C++ a member with an in-class initializer must be const

Question

I am trying to create a static string in my class: (in my header file)

static string description = "foo";

but I'm getting this error:

IntelliSense: a member with an in-class initializer must be const

if I change it to this:

static const string description = "foo";

I get this error instead:

IntelliSense: a member of type "const std::string" cannot have an in-class initializer

What did I do wrong?

Answer 1

What you can do is declare the string in the header and initialize it in your .cpp.

in MyClass.h

#include <string>
class MyClass
{
  static std::string foo;
}

in MyClass.cpp

#include "MyClass.h"
std::string MyClass::foo = "bar"

Answer 2

Ignoring the specific error message the core issue is that you are trying to initialize a static member attribute in the declaration while in general that should be done in the definition.

// header
struct test {
  static std::string x;
};
// single cpp
std::string test::x = "foo";

Now back to the error messages. There is an exception in the C++03 standard that allows to provide an initializer to the declaration of a constant integral type, so that the value can be visible in all translation units that include the header and can thus be used as a constant expression:

// header
struct test {
   static const int size = 10;
};
// some translation unit can do
struct A {
   int array[test::size];
};

If the value was defined in the definition of the variable, then the compiler could only use it in that single translation unit. It seems that your compiler is doing two tests, one for const-ness and a separate one for the integral part and thus two error messages.

Another thing that might affected that design in the compiler is that the C++11 standard allows for initializers in the declaration of non-static members of a class, which will then be used in the initializer list of each constructor that does not provide a value for that field:

struct test {
   int a = 10;
   int b = 5;
   test() : a(5) // b(5) implicitly generated
   {} 
};

This is unrelated to your particular issue, as your member is static but it probably explains why the tests in the compiler were split as they are.

Answer 3

Seperate the declaration from the definition. In the header file, do this:

static string description;

And then in exactly one translation unit (one CPP file), do this:

string type::description = "foo";

Answer 4

I don't know what exactly you need between a static member and a constant member. A static member will be associated with the class itself and not with the instances, and a constant member is a associated with the instance and will be constant.

However, this is a possible duplicate of this

Regards

Answer 5

I believe from c++11 you could use the constexpr

class {
//...more code here
public: static constexpr float X = 1.2f;
}