There's the main.cpp that has lots of Log prinkled wherever:
#include "utils.hpp"
...//some code
int main(){
int a = 0;
int b = 0;
util::LogClass::Log("Initial","something);
//some more code
util::LogClass::Log("Mid","something");
//some more code
util::LogClass::Log("Middle","something");
}
And the LogClass is defined like this in utils.hpp:
namespace util{
class LogClass{
public:static bool LOG_ENABLED;
public: static void Log(std::string tag, std::string message){
if(LOG_ENABLED){
std::cerr << tag+": "+message <<std::endl;}
}
}
bool LogClass::LOG_ENABLED=true;
}
I was thinking I would be able to do this in main.cpp:
#include "utils.cpp"
util::LogClass::LOG_ENABLE=false;
int main(){ //the previous main function}
*the above code actuallly gives an error saying: redefinition of ‘bool util::LogClass::LOG_ENABLED’
bool util::LogClass::LOG_ENABLE=false *
but, if I move it inside the main:
#include "utils.cpp"
int main(){ util::LogClass::LOG_ENABLED=false; //the previous main function}
then the code compiles fine. So my question is why can't I enable it outside the main() function even if it is a static member, and why does the (g++) compiler takes it as a redefinition?
You can only statically initialize a variable at the point where it is getting defined. The initialization inside the main function is dynamic, so that's fine.
I agree that the compiler error is weird though - the compiler might be trying to auto-deduct the "missing" type that should be there for a redefinition.
[my code is quiet big, so I only pasted parts that seemed relevant to me, just tell me if you need to see more of it, thanks]
I defined a constant in a class (Animation) but now I split the class in two (AnimationsDefinition & AnimationsInstance), I can't use that constant from one class to the other
I included AnimationsDefinition.h before AnimationsInstance.h
common.h
#include "animationsDefinition.h"
#include "animationsInstance.h"
the compiler complains the constant cannot be evaluated
the below code used to be working, but now I use it in another class, it does not work anymore
the error I get :
error C2131: expression did not evaluate to a constant
note: failure was caused by non-constant arguments or reference to a non-constant symbol
note: see usage of 'EAST'
AnimationsDefinition.cpp
#include "common.h"
const int AnimationsDefinition::WEST = 0;
const int AnimationsDefinition::SOUTH = 1;
const int AnimationsDefinition::NORTH = 2;
const int AnimationsDefinition::EAST = 3;
...
AnimationsDefinition.h
#pragma once
class AnimationsDefinition
{
public:
static const int WEST;
static const int SOUTH;
static const int NORTH;
static const int EAST;
...
AnimationsInstance.cpp
#include "common.h"
void AnimationsInstance::update(float tpf)
{
switch (direction)
{
case AnimationsDefinition::EAST: <<<<<<<<< compilation error
{
...
Any help appreciated
regards
You can initialize the static const variables within the class itself.
class AnimationsDefinition
{
public:
static const int WEST = 0;
static const int SOUTH = 1;
static const int NORTH = 2;
static const int EAST = 3;
};
However, you would want to define them instead properly if you want to avoid ODR violations.
So use constexpr specifier which implies inline for static data members. And inline static data members can be defined in the class definition itself.
class AnimationsDefinition
{
public:
static constexpr int WEST = 0;
static constexpr int SOUTH = 1;
static constexpr int NORTH = 2;
static constexpr int EAST = 3;
};
As per [class.static.data]/3:
An inline static data member may be defined in the class definition and may specify a brace-or-equal-initializer.
Const variables can be used as constant expressions only after their initialiser has been encountered. In this case, that can be achieved by moving the initialisers into the header.
Note that moving the initialiser into the declaration within the class definition in the header does not make the declaration into a definition of the variable. The definitions must still remain in the source file, just without the initialisers.
Ok I found out I could define these in the class itself since they are static
You can define them in the class itself (or the header in general) only if you declare them inline.
However, it is more conventional to use (scoped) enumerations for switch cases.
What is the best way to initialize a private, static data member in C++? I tried this in my header file, but it gives me weird linker errors:
class foo
{
private:
static int i;
};
int foo::i = 0;
I'm guessing this is because I can't initialize a private member from outside the class. So what's the best way to do this?
The class declaration should be in the header file (Or in the source file if not shared).
File: foo.h
class foo
{
private:
static int i;
};
But the initialization should be in source file.
File: foo.cpp
int foo::i = 0;
If the initialization is in the header file then each file that includes the header file will have a definition of the static member. Thus during the link phase you will get linker errors as the code to initialize the variable will be defined in multiple source files.
The initialisation of the static int i must be done outside of any function.
Note: Matt Curtis: points out that C++ allows the simplification of the above if the static member variable is of const integer type (bool, char, char8_t [since C++20], char16_t, char32_t, wchar_t, short, int, long, long long, or any implementation-defined extended integer types, including any signed, unsigned, and cv-qualified variants.). You can then declare and initialize the member variable directly inside the class declaration in the header file:
class foo
{
private:
static int const i = 42;
};
For a variable:
foo.h:
class foo
{
private:
static int i;
};
foo.cpp:
int foo::i = 0;
This is because there can only be one instance of foo::i in your program. It's sort of the equivalent of extern int i in a header file and int i in a source file.
For a constant you can put the value straight in the class declaration:
class foo
{
private:
static int i;
const static int a = 42;
};
Since C++17, static members may be defined in the header with the inline keyword.
http://en.cppreference.com/w/cpp/language/static
"A static data member may be declared inline. An inline static data member can be defined in the class definition and may specify a default member initializer. It does not need an out-of-class definition:"
struct X
{
inline static int n = 1;
};
For future viewers of this question, I want to point out that you should avoid what monkey0506 is suggesting.
Header files are for declarations.
Header files get compiled once for every .cpp file that directly or indirectly #includes them, and code outside of any function is run at program initialization, before main().
By putting: foo::i = VALUE; into the header, foo:i will be assigned the value VALUE (whatever that is) for every .cpp file, and these assignments will happen in an indeterminate order (determined by the linker) before main() is run.
What if we #define VALUE to be a different number in one of our .cpp files? It will compile fine and we will have no way of knowing which one wins until we run the program.
Never put executed code into a header for the same reason that you never #include a .cpp file.
Include guards (which I agree you should always use) protect you from something different: the same header being indirectly #included multiple times while compiling a single .cpp file.
With a Microsoft compiler[1], static variables that are not int-like can also be defined in a header file, but outside of the class declaration, using the Microsoft specific __declspec(selectany).
class A
{
static B b;
}
__declspec(selectany) A::b;
Note that I'm not saying this is good, I just say it can be done.
[1] These days, more compilers than MSC support __declspec(selectany) - at least gcc and clang. Maybe even more.
int foo::i = 0;
Is the correct syntax for initializing the variable, but it must go in the source file (.cpp) rather than in the header.
Because it is a static variable the compiler needs to create only one copy of it. You have to have a line "int foo:i" some where in your code to tell the compiler where to put it otherwise you get a link error. If that is in a header you will get a copy in every file that includes the header, so get multiply defined symbol errors from the linker.
If you want to initialize some compound type (f.e. string) you can do something like that:
class SomeClass {
static std::list<string> _list;
public:
static const std::list<string>& getList() {
struct Initializer {
Initializer() {
// Here you may want to put mutex
_list.push_back("FIRST");
_list.push_back("SECOND");
....
}
}
static Initializer ListInitializationGuard;
return _list;
}
};
As the ListInitializationGuard is a static variable inside SomeClass::getList() method it will be constructed only once, which means that constructor is called once. This will initialize _list variable to value you need. Any subsequent call to getList will simply return already initialized _list object.
Of course you have to access _list object always by calling getList() method.
C++11 static constructor pattern that works for multiple objects
One idiom was proposed at: https://stackoverflow.com/a/27088552/895245 but here goes a cleaner version that does not require creating a new method per member.
main.cpp
#include <cassert>
#include <vector>
// Normally on the .hpp file.
class MyClass {
public:
static std::vector<int> v, v2;
static struct StaticConstructor {
StaticConstructor() {
v.push_back(1);
v.push_back(2);
v2.push_back(3);
v2.push_back(4);
}
} _staticConstructor;
};
// Normally on the .cpp file.
std::vector<int> MyClass::v;
std::vector<int> MyClass::v2;
// Must come after every static member.
MyClass::StaticConstructor MyClass::_staticConstructor;
int main() {
assert(MyClass::v[0] == 1);
assert(MyClass::v[1] == 2);
assert(MyClass::v2[0] == 3);
assert(MyClass::v2[1] == 4);
}
GitHub upstream.
Compile and run:
g++ -ggdb3 -O0 -std=c++11 -Wall -Wextra -pedantic -o main.out main.cpp
./main.out
See also: static constructors in C++? I need to initialize private static objects
Tested on Ubuntu 19.04.
C++17 inline variable
Mentioned at: https://stackoverflow.com/a/45062055/895245 but here is a multifile runnable example to make it even clearer: How do inline variables work?
This awesome C++17 feature allow us to:
conveniently use just a single memory address for each constant
store it as a constexpr: How to declare constexpr extern?
do it in a single line from one header
main.cpp
#include <cassert>
#include "notmain.hpp"
int main() {
// Both files see the same memory address.
assert(¬main_i == notmain_func());
assert(notmain_i == 42);
}
notmain.hpp
#ifndef NOTMAIN_HPP
#define NOTMAIN_HPP
inline constexpr int notmain_i = 42;
const int* notmain_func();
#endif
notmain.cpp
#include "notmain.hpp"
const int* notmain_func() {
return ¬main_i;
}
Compile and run:
g++ -c -o notmain.o -std=c++17 -Wall -Wextra -pedantic notmain.cpp
g++ -c -o main.o -std=c++17 -Wall -Wextra -pedantic main.cpp
g++ -o main -std=c++17 -Wall -Wextra -pedantic main.o notmain.o
./main
GitHub upstream.
I don't have enough rep here to add this as a comment, but IMO it's good style to write your headers with #include guards anyway, which as noted by Paranaix a few hours ago would prevent a multiple-definition error. Unless you're already using a separate CPP file, it's not necessary to use one just to initialize static non-integral members.
#ifndef FOO_H
#define FOO_H
#include "bar.h"
class foo
{
private:
static bar i;
};
bar foo::i = VALUE;
#endif
I see no need to use a separate CPP file for this. Sure, you can, but there's no technical reason why you should have to.
You can also include the assignment in the header file if you use header guards. I have used this technique for a C++ library I have created. Another way to achieve the same result is to use static methods. For example...
class Foo
{
public:
int GetMyStatic() const
{
return *MyStatic();
}
private:
static int* MyStatic()
{
static int mStatic = 0;
return &mStatic;
}
}
The above code has the "bonus" of not requiring a CPP/source file. Again, a method I use for my C++ libraries.
The linker problem you encountered is probably caused by:
Providing both class and static member definition in header file,
Including this header in two or more source files.
This is a common problem for those who starts with C++. Static class member must be initialized in single translation unit i.e. in single source file.
Unfortunately, the static class member must be initialized outside of the class body. This complicates writing header-only code, and, therefore, I am using quite different approach. You can provide your static object through static or non-static class function for example:
class Foo
{
// int& getObjectInstance() const {
static int& getObjectInstance() {
static int object;
return object;
}
void func() {
int &object = getValueInstance();
object += 5;
}
};
I follow the idea from Karl. I like it and now I use it as well.
I've changed a little bit the notation and add some functionality
#include <stdio.h>
class Foo
{
public:
int GetMyStaticValue () const { return MyStatic(); }
int & GetMyStaticVar () { return MyStatic(); }
static bool isMyStatic (int & num) { return & num == & MyStatic(); }
private:
static int & MyStatic ()
{
static int mStatic = 7;
return mStatic;
}
};
int main (int, char **)
{
Foo obj;
printf ("mystatic value %d\n", obj.GetMyStaticValue());
obj.GetMyStaticVar () = 3;
printf ("mystatic value %d\n", obj.GetMyStaticValue());
int valMyS = obj.GetMyStaticVar ();
int & iPtr1 = obj.GetMyStaticVar ();
int & iPtr2 = valMyS;
printf ("is my static %d %d\n", Foo::isMyStatic(iPtr1), Foo::isMyStatic(iPtr2));
}
this outputs
mystatic value 7
mystatic value 3
is my static 1 0
Also working in privateStatic.cpp file :
#include <iostream>
using namespace std;
class A
{
private:
static int v;
};
int A::v = 10; // possible initializing
int main()
{
A a;
//cout << A::v << endl; // no access because of private scope
return 0;
}
// g++ privateStatic.cpp -o privateStatic && ./privateStatic
What about a set_default() method?
class foo
{
public:
static void set_default(int);
private:
static int i;
};
void foo::set_default(int x) {
i = x;
}
We would only have to use the set_default(int x) method and our static variable would be initialized.
This would not be in disagreement with the rest of the comments, actually it follows the same principle of initializing the variable in a global scope, but by using this method we make it explicit (and easy to see-understand) instead of having the definition of the variable hanging there.
One "old-school" way to define constants is to replace them by a enum:
class foo
{
private:
enum {i = 0}; // default type = int
enum: int64_t {HUGE = 1000000000000}; // may specify another type
};
This way doesn't require providing a definition, and avoids making the constant lvalue, which can save you some headaches, e.g. when you accidentally ODR-use it.
Here are all possibilities and errors in one simple example ...
#ifndef Foo_h
#define Foo_h
class Foo
{
static const int a = 42; // OK
static const int b {7}; // OK
//static int x = 42; // ISO C++ forbids in-class initialization of non-const static member 'Foo::x'
//static int y {7}; // ISO C++ forbids in-class initialization of non-const static member 'Foo::x'
static int x;
static int y;
int m = 42;
int n {7};
};
// Foo::x = 42; // error: 'int Foo::x' is private
int Foo::x = 42; // OK in Foo.h if included in only one *.cpp -> *.o file!
int Foo::y {7}; // OK
// int Foo::y {7}; // error: redefinition of 'int Foo::y'
// ONLY if the compiler can see both declarations at the same time it,
// OTHERWISE you get a linker error
#endif // Foo_h
But better place this in Foo.cpp. This way you can separately compile each file and link them later, otherwise Foo:x will be present in multiple object files and cause a linker error. ...
// Foo::x = 42; // error: 'int Foo::x' is private, bad if Foo::X is public!
int Foo::x = 42; // OK in Foo.h if included in only one *.cpp -> *.o file!
int Foo::y {7}; // OK
Does this serves your purpose?
//header file
struct MyStruct {
public:
const std::unordered_map<std::string, uint32_t> str_to_int{
{ "a", 1 },
{ "b", 2 },
...
{ "z", 26 }
};
const std::unordered_map<int , std::string> int_to_str{
{ 1, "a" },
{ 2, "b" },
...
{ 26, "z" }
};
std::string some_string = "justanotherstring";
uint32_t some_int = 42;
static MyStruct & Singleton() {
static MyStruct instance;
return instance;
}
private:
MyStruct() {};
};
//Usage in cpp file
int main(){
std::cout<<MyStruct::Singleton().some_string<<std::endl;
std::cout<<MyStruct::Singleton().some_int<<std::endl;
return 0;
}
I just wanted to mention something a little strange to me when I first encountered this.
I needed to initialize a private static data member in a template class.
in the .h or .hpp, it looks something like this to initialize a static data member of a template class:
template<typename T>
Type ClassName<T>::dataMemberName = initialValue;
I am trying to reach a static variable declared in MyClass.h from MyClass.cpp. But I get following errors.
I made a research but still have no clue why my code does not compile. I use visual studio 2013.
MyClass.h
#ifndef __MyClass_h_
#define __MyClass_h_
class MyClass {
static int x;
public:
static int y;
};
#endif
MyClass.cpp
#include "MyClass.h"
void MyClass::sor() (const string& var1, const unsigned count) const {
// What goes here? See below for what I have tried
}
So, if I use:
int MyClass::x=8;
This says int MyClass::x redefinition and MyClass::x 'MyClass::x' : definition or redeclaration illegal in current scope
If I use:
MyClass::x=8;
This gives the error 1 unresolved external.
If I use:
MyClass::y=8;
This also gives the error 1 unresolved external.
If I use:
int MyClass::y=8;
This says int MyClass::y redefinition and 'MyClass::y' : definition or redeclaration illegal in current scope
You need to understand you don't have a static variable in a header, how other answers suggest. You have a static member of a class, which is perfectly fine.
In order to access it you write: MyClass::x. You need to initialize it also.
Unrelated to the static member, you need to declare the method also:
header:
#ifndef __MyClass_h_
#define __MyClass_h_
class MyClass {
static int x;
public:
static int y;
void sor() (const string& var1, const unsigned count) const;
};
#endif
source file:
#include "MyClass.h"
int MyClass::x = 0; // intialization
void MyClass::sor() (const string& var1, const unsigned count) const {
MyClaxx::x = 11; // access it
}
You have a declaration for the static variable but don't have the definition for it. In order to use static class members you should define them in the corresponding translation unit. You can't do it in the header file because it will violate ODR(one definition rule) so you should define it in the .cpp file to confrom the aforementioned rule.
So you have to put int MyClass::x = 0; in your cpp file in global scope(or under the namespace if you have one) to get it working. Note, that you could use whatever value insted of 0 or even didn't provide any(in this case it will be 0 anyway, due to special treatment of global(static) variables.)
When static variable is declared in a header file is its scope limited to .h file or across all units.
Refer here for source
This is simple. When you declare a static variable in a header file, it's scope is limited to header file. When you going to use that static variable in a .cpp file you getting an error like this. It is because you didn't give the definition of the static variable. So in any of the .cpp file you need to give the definition of static variable. ex :
.h file
class MyClass {
static int x;
.....
}
Top of the .cpp file you should define the static variable like this.
#include "MyClass.h"
int MyClass::x = 0 ;
void MyClass::sor() (const string& var1, const unsigned count) const {
MyClass::x = 8 ; // you can access without any issue
}
One would think that this simple piece of code should compile easily:
#include <utility>
struct Q {
static const int X = 0;
};
int main() {
std::pair<int, int>(Q::X, 0);
return 0;
}
However, when I compile this with g++ using the default settings (cygwin gcc 4.5.3) it throws a linker error at me:
undefined reference to `Q::X'
I'm totally stumped here -- MSVC compiles this just fine yet my gcc does not. (MinGW also works fine.)
The linker fails to find definition of Q::X.
This is probably because std::pair<> constructor takes arguments as references to const, and Q::X is an l-value, hence it requires an external definition of Q::X to be able to refer to that.
A portable fix:
int const Q::X;
You only declare X but you must also define it, which, for static definitions must happen outside the class
struct Q {
static const int X = 0;
};
int Q::X = 0;
#Maxim Yegorushkin (Beat me to it!) Here is a relevant link that might help.
But your problem is that the int is never externally defined. If X doesn't have to be const, you can initialize it outside the struct in a similar manner. The main reason is because X is static...
struct Q {
static const int X;
};
int const Q::X = 0;
struct Q {
static int X;
};
int Q::X = 0;