// in someFile.h or someFile.cpp
TEST()
{
"example test", []
{
EXPECT(0 == 1);
}
}
TEST_END()
// in main.cpp
int main() { ssvu::Test::runAllTests(); }
#define TEST() static RunOnCtor UNIQUENAME(__LINE__) { []{ getStaticTests().push_back({
#define TEST_END() });}};
struct RunOnCtor { RunOnCtor(std::function<void()> f) { f(); } };
I've created some unit testing macros that work both in header (useful for my header-only libraries) and source files.
TEST() creates a static instance of RunOnCtor, which, on construction, executes a lambda which inserts the test lambda into a std::vector of tests. runAllTests() runs every test in that vector.
I have created a DISABLE_TEST define that simply puts return; at the beginning of the RunOnCtor lambda, so that no tests are added to the internal vector. However, I would like to prevent unnecessary construction of static objects when tests are disabled.
Is there a way to completely ignore everything between TEST() and TEST_END()? Something like commenting it out. Or would emptying out RunOnCtor (empty struct) make the compiler avoid static constructions?
I think this should work:
#define TEST() struct UNIQUENAME(__LINE__) { void f() {
#define TEST_END() } };
The following is based on #Angew 's answer but uses standard preprocessor definitions. The class then defines a function within the class declaration, which in C++ forces it to be inline. Because the class is never instantiated and the function is never called, no code is ever generated.
#define CAT(a, ...) PCAT(a, __VA_ARGS__)
#define PCAT(a, ...) a ## __VA_ARGS__
#define TEST() class CAT(Obfuscate_, __LINE__) { void f() {
#define TEST_END() } };
int c;
TEST()
int a = 7;
int b = a * 17;
c = b + 4;
return;
TEST_END()
Here's Godbolt's handy-dandy compiler explorer to prove that GCC, CLANG, and ICC produce no code for the above: https://godbolt.org/g/BXKDNF
Related
For simplicity, I will omit things like proper typedefs to opaque structs instead of void *, Windows calling conventions, fixed integer types, etc.
Suppose I have the following files:
CApi.h -- Shared library header with C linkage for portability and hiding proprietary code.
#define LIB_API // library import/export details
extern "C" {
typedef int error;
error LIB_API lib_foo_create(void ** foo);
error LIB_API lib_foo_destroy(void * foo);
error LIB_API lib_foo_func(void * foo, int * out);
error LIB_API lib_bar_create(void ** bar);
error LIB_API lib_bar_destroy(void * bar);
error LIB_API lib_bar_func(void * bar, int * out); // Suppose this internally uses Foo::func from CxxApi.hpp with C++17
} // extern C
CxxApi.hpp -- Header only wrapper to simplify API usage.
#include "CApi.h"
namespace lib {
namespace detail {
template < typename Return = void, typename Func, typename... Args >
Return c_api(Func func)
{
// Not sure how this affects ODR if client and provider code use different versions,
// but neither see each other's code usage.
#if __cplusplus >= 201703L // C++17
// more efficient implmentation (e.g. fold expressions)
#else
// fallback implmentation (e.g. recursion)
#endif
}
} // namespace lib::detail
class Foo
{
public:
Foo() { detail::c_api(lib_foo_create, &handle_); }
~Foo() { detail::c_api(lib_foo_destroy, handle_); }
int func() { return detail::c_api<int>(lib_foo_func, handle_); }
private:
void * handle_;
};
struct Bar
{
public:
Bar() { detail::c_api(lib_bar_create, &handle_); }
~Bar() { detail::c_api(lib_bar_destroy, handle_); }
int func() { return detail::c_api<int>(lib_bar_func, handle_); }
private:
void * handle_;
};
} // namespace lib
If I, the API provider, compile this using C++17, but the client using the API uses C++11, is ODR violated with lib::detail::c_api?
I believe it is not because lib_bar_func's definition is in a different translation unit than client code, but I am not positive.
I am trying to export a function template with a static variable in the definition.
.dll/Foo.h:
#ifdef _DLL
#define API __declspec(dllexport)
#else
#define API __declspec(dllimport)
#endif
class API Foo
{
public:
template<typename T>
static T& Get()
{
static T _instance;
return _instance;
}
static void Set();
}
I want the calls made by the .dll and .exe to refer to the same "_instance" object. I know I can do that by defining the static variable in the .cpp. But in this case i am dealing with templates so i am kind of stuck.
Edit:
Example of what is happening..
.dll/Foo.cpp:
void Foo::Set()
{
Foo::Get<int>() = 10;
}
.exe/main.cpp:
int main()
{
auto & x = Foo::Get<int>();
x = 3;
std::cout << x; // 3
Foo::Set();
std::cout << x; // 3 (I want it to be 10)
}
you need separate mark every template with API (__declspec(dllexport) or __declspec(dllimport)) and not inline it in class code.
Foo.h file is:
#ifdef _DLL
#define API __declspec(dllexport)
#else
#define API __declspec(dllimport)
#endif
class API Foo
{
public:
template<typename T>
API static T& Get();
static void Set();
};
note that we separate mark Get() with API despite all Foo class also marked with API (class mark have no effect on template function by fact, so need it mark separate). and no implementation of Get here - exported functions anyway can not be inline.
so dll code (Foo.cpp) must look like:
#include "foo.h"
template<typename T>
API T& Foo::Get()
{
__pragma(message("__imp_" __FUNCDNAME__)) // for debug
static T _instance;
return _instance;
}
void Foo::Set()
{
Foo::Get<int>() = 10;
}
note we again explicitly use API (__declspec(dllexport)) in implementation of function body. this is critical important - compiler not warning you, if you skip API here, but without this - Get will be not exported.
for sure that at this point all correct - copy string produced by __pragma(message("__imp_" __FUNCDNAME__)) (it will be look like __imp_??$Get#H#Foo##SAAEAHXZ) and search exactly (symbol to symbol) this string in created .lib file - after you build dll. if it exist - all ok, otherwise no sense continue (with exe)
and in exe :
#include "../foo_dll/foo.h"
Foo::Get<int>() = 3;
Foo::Set();
if (Foo::Get<int>() != 10)
{
__debugbreak();
}
I'm doing functional testing on c code. Have include .c file in test .cpp file so far, everything works well. But I need to include the same .c file in another .cpp file where I do other unit tests. Then I get problems that is already defined. Because I already include it in the first file cpp.
If merge all the test methods into one cpp file then it works well. If keep all unit test in one file get to big handle so I need to split up diffrent files in same project it also create help functions for secure that functions get in capsules.
This not a normal LNK2005 because I can not declare the variable and functions in .h: as extern BOOL MyBool; and then assign to it in a .c or .cpp file. Because need include c file as I do unit test this function. Also I can't or should avoid do any changes .c.
I looking way to keep include of .c local not effect other files in same project.
source.h
#ifndef SOURCE_H_
#define SOURCE_H_
#include "../car.h"
enum INITIALMODE {
INITIALMODE_NOT_POSITIONING, // 0
INITIALMODE_NO_DRIVER_INPUT, // 1
INITIALMODE_POSITION_LOW_POSITION, // 2
INITIALMODE_POSITION_STANDARD_POSITION, // 3
INITIALMODE_POSITION_HIGH_POSITION // 4
};
void initMotor(void);
#endif
source.c
/* Compiler include files */
#pragma once
#include "positioning.h"
#include "api.h"
#include "drive.h"
#include "types.h"
static void updateTarget(void);
static SWord getMax(UWord Limit, UWord Aux);
static DWord getHeight(void);
static Bool isMode(void);
static Bool isExiting(void);
#define cMaxHeight 100 * Profile.s.Max /* m -> mm */
void initMotor(void)
{
// do something
}
static void updatePositioning(void)
{
// do something
}
Test files look like this, however, this is very scaled for making exemple small.
UnitTest.cpp and UnitTest2.cpp
#include "CppUnitTest.h"
#ifndef UNIT_TEST
#define UNIT_TEST
using namespace Microsoft::VisualStudio::CppUnitTestFramework;
namespace Test
{
extern "C" {
#include "../../Test/source.h"
#include "../../Test/source.c"
}
TEST_CLASS(UnitTest_1)
{
public:
TEST_METHOD(Test_1)
{
// Verify that the initial state is as expected.
initTest();
//Expected value
UWord Expected = 500
//Trigger to execute.
UWord Test = updatePositioning();
// Verify that the results are as expected.
Assert::AreEqual(Expected, Test);
}
};
}
#endif
You should never include a .C or .CPP file.
However, you can compile C code in C++. Here's an example based on the information you gave in your initial post.
YourCCode.h
#ifndef YourCCode_H
#define YourCCode_H
int FunctionToTest(int SomeParams);
int TestStaticFunctions(int SomeParam1, int SomeParam2);
#endif // YourCCode_H
YourCCode.c
#include "YourCCode.h"
static int SomeStaticFunction(int Param1, int Param2)
{
return Param1 + Param2; // that needs extensive testing, obviously.
}
int FunctionToTest(int SomeParams)
{
return SomeStaticFunction(SomeParams, 1);
}
int TestStaticFunctions(int SomeParam1, int SomeParam2)
{
return SomeStaticFunction(SomeParam1, SomeParam2);
}
UnitTest1.cpp
extern "C" {
#include "YourCCode.h"
}
bool TestFunction(int Value)
{
return (FunctionToTest(Value) == Value+1);
}
UnitTest2.cpp
extern "C" {
#include "YourCCode.h"
}
void AnotherTestFunction(int Val, int Val2)
{
int newValue = TestStaticFunctions(Val, Val2);
ASSERT(newValue == Val+Val2);
}
Then compile your CPP and C file.
After you clarified your intent, I realized you're trying to test static functions from another unit. Static function, by definition, are only available to other functions in the same translation unit. This is mostly use as a protection to prevent programmers to call some functions without knowing how to validate their inputs, know the call order, etc...
My best bet here is either you choose that your functions are not static and you can test them from outside your translation unit, or you implement the test functions inside the translation unit containing those static functions. I would recommend the latter as a translation unit should know (architecturally speaking) how to test their own features.
As a third solution, if you don't have any control over the content of the C file (but since you have that C file, I doubt it), you could have a proxy CPP file that includes the C file, and create a proxy call for each static call.
That is a ugly hack, though, and could break very easily if the C file ever gets updated, so I'm advising against it.
Here's a quick example :
YourCCode.h
#ifndef YourCCode_H
#define YourCCode_H
void SomeFunction(void);
#endif // YourCCode_H
YourCCode.c
#include "YourCCode.h"
static int AddSomething(int Param1, int Param2)
{
return Param1 + Param2;
}
static int SubtractSomething(int Param1, int Param2)
{
return Param1 - Param2;
}
void SomeFunction(void)
{
// code meant to be called externally.
}
ProxyTestCode.hpp
bool TestAddSomething(void);
bool TestSubtractSomething(void);
ProxyTestCode.cpp
extern "C" {
#include "YourCCode.h"
#include "YourCCode.c"
}
bool TestAddSomething(void)
{
return (AddSomething(2,2) == 4);
}
bool TestSubtractSomething(void)
{
return (AddSomething(2,2) == 0);
}
UnitTest1.cpp
#include "ProxyTestCode.hpp"
void TestAdd(void)
{
ASSERT(TestAddSomething());
}
UnitTest2.cpp
#include "ProxyTestCode.hpp"
void TestSubtract(void)
{
ASSERT(TestSubtractSomething());
}
If you do that, don't compile your C file in your project.
I have some well-formed code looks like this:
NAMESPACE_BEGIN(Foo)
inline void test() {
string s = xxx;
}
NAMESPACE_END(Foo)
So, is there any portable tricks by using the NAMESPACE_BEGIN() macro to obtain namespace name "Foo" in test()?
I'm thinking of something like this, but it would surely cause symbol redefinition:
#define NAMESPACE_BEGIN(x) \
namespace x { \
inline const char *_curNamespace() { \
return #x; \
}
#define NAMESPACE_END(x) \
}
There's also a workaround looks like this, but that's not very convenient
#define NAMESPACE_NAME Foo
// using a header file so that we can use #ifdef guard
#include "MyNamespaceBegin.h"
...
#include "MyNamespaceEnd.h"
EDIT:
Why I need this:
I'm using much of macro to generate codes to achieve some
dynamic reflection logic (yes, not static template reflection),
it's all right within class scope by using static member function,
but does not work for namespaces
Why not to manually declare the name getter once:
What I want is something like this:
// the global default version
const char *_curNamespace() {return "";}
namespace X {
// the local namespace version
const char *_curNamespace() {return "X";}
// some verbose reflection register code
...
registerSomething(_curNamespace());
...
}
Of course, all of the verbose register code should be generated by macro
And, app level user should not care about the _curNamespace(),
so, I want to simplify the user's usage,
by using a custom NAMESPACE_BEGIN(xxx) macro at any case
If you are still curious about what I'm doing,
check this: https://github.com/ZFFramework/ZFFramework
I'm using lots of tricks to achieve fully dynamic reflection in pure C++,
to achieve some of my fancy thoughts,
for now, this project is just for fun,
I have no idea whether it has practicability
EDIT2:
For now, I think the best workaround should be like this:
#define NAMESPACE_BEGIN(ns) \
namespace ns { \
extern const char *__curNS();
#define NAMESPACE_END(ns) \
}
#define NAMESPACE_REG(ns) \
const char *__curNS() {return #ns;}
app level users still only need to care about NAMESPACE_BEGIN
NAMESPACE_REG must be declared exactly once, in source file
if not, undefined symbol would happen
if more than once, duplicated symbol would happen
although it's annoying and sometimes you need additional source file
to hold the NAMESPACE_REG,
the strict rule should prevent user from forgetting the ugly workaround
You are making much fuss over something that is trivial to implement.
First of all, use of NAMESPACE_BEGIN and NAMESPACE_END seems unnecessary to me. I don't see how that is more readable or useful than
namespace Foo
{
}
If getting the name of the namespace is important/useful, add a trivial function.
namespace Foo
{
inline std::string get_name() { return "Foo"; }
}
Small sized real world applications need thousands of lines of code. Large sized real world applications need millions of lines of code. From that perspective, implementing a one line inline function is a very minor task.
This solution employs a bit of preprocessor magic and has these features:
Namespace is mentioned only once
Access to a macro containing the unquoted name
Access to a macro containing the quoted name
Support for repeating the same namespace
Support for different namespaces
Misuse of the BEGIN/END macros is detected
Cleanup, i.e. no extra macros defined outside the BEGIN/END block
It does not support nested namespaces.
Example of usage:
#include "framework.hpp"
#define NAMESPACE_NAME Foo
#include NAMESPACE_BEGIN
// Here you have access to NAMESPACE_NAME (unquoted, i.e. Foo)
// and also to NAMESPACE_NAME_STRING (quoted, i.e. "Foo")
#include NAMESPACE_END
// NAMESPACE_NAME and NAMESPACE_NAME_STRING do not exist
// outside the block, so they cannot be misused
// Different namespaces in the same TU are supported
#define NAMESPACE_NAME Bar
#include NAMESPACE_BEGIN
inline std::string f()
{
return NAMESPACE_NAME_STRING;
}
#include NAMESPACE_END
// Repeating the same namespace is also supported
#define NAMESPACE_NAME Foo
#include NAMESPACE_BEGIN
inline std::string f()
{
return NAMESPACE_NAME_STRING;
}
#include NAMESPACE_END
The implementation follows:
framework.hpp
#pragma once
#define NAMESPACE_BEGIN "framework_namespace_begin.hpp"
#define NAMESPACE_END "framework_namespace_end.hpp"
framework_namespace_begin.hpp
#ifndef NAMESPACE_NAME
#error "NAMESPACE_NAME not defined"
#endif
#define NAMESPACE_IN_NAMESPACE 1
#define NAMESPACE_NAME_DO_STR(X) #X
#define NAMESPACE_NAME_STR(X) NAMESPACE_NAME_DO_STR(X)
#define NAMESPACE_NAME_STRING NAMESPACE_NAME_STR(NAMESPACE_NAME)
namespace NAMESPACE_NAME {
framework_namespace_end.hpp
#ifndef NAMESPACE_IN_NAMESPACE
#error "NAMESPACE_IN_NAMESPACE not defined"
#endif
}
#undef NAMESPACE_NAME
#undef NAMESPACE_NAME_STRING
#undef NAMESPACE_IN_NAMESPACE
You know what? I think I might just have a viable solution for this. It's actually very simple, and it's very close to the OP's original suggestion (which really only had the problem of a potential duplicate definition if you wanted to open the namespace twice in the same translation unit). You just have to think a bit laterally and not be too precious about seeing your namespaces being bracketed by macros instead of curly braces.
So let me just lay it out here, because there's really nothing to it, and then I'll explain why I personally happen to like it.
Code:
Macros:
#define DECLARE_NAMESPACE(ns) \
namespace ns {\
static constexpr const char *_curNamespace = #ns; \
}
#define BEGIN_NAMESPACE(ns) \
namespace ns { \
static_assert (ns::_curNamespace, "BEGIN_NAMESPACE: namespace has not been declared");
#define END_NAMESPACE }
Sample code:
#include <iostream>
DECLARE_NAMESPACE (Foo)
BEGIN_NAMESPACE (Foo)
void print_namespace_name () { std::cout << _curNamespace << "\n"; }
END_NAMESPACE
BEGIN_NAMESPACE (Foo)
void another_print_namespace_name () { std::cout << _curNamespace << "\n"; }
END_NAMESPACE
DECLARE_NAMESPACE (Bar)
BEGIN_NAMESPACE (Bar)
void print_namespace_name () { std::cout << _curNamespace << "\n"; }
DECLARE_NAMESPACE (BarBar)
BEGIN_NAMESPACE (BarBar)
void print_namespace_name () { std::cout << _curNamespace << "\n"; }
END_NAMESPACE
END_NAMESPACE
int main ()
{
Foo::print_namespace_name ();
Foo::another_print_namespace_name ();
Bar::print_namespace_name ();
Bar::BarBar::print_namespace_name ();
}
Output:
Foo
Foo
Bar
BarBar
Now this is obviously very straightforward to implement and also easy to use and has no obvious limitations. In particular, it can handle nested namespaces (as shown in the code above) and opening a namespace twice in the same compilation unit also works (again, this is shown in the code snippet).
But, but, but, don't we still have to type in the name of the namespace twice, and wasn't that the very thing we were trying to avoid to eliminate typos?
Well, sure, we have to type the name in twice, but so what, live with it. Point is, with this particular set of macros, the compiler will now catch any typos for us. Let's prove that by deliberately putting one in. So this:
DECLARE_NAMESPACE Whoops
BEGIN_NAMESPACE whoops
END_NAMESPACE
Generates this (I couldn't find a better way of formulating the static_assert, sorry):
prog.cc:12:24: error: '_curNamespace' is not a member of 'whoops'
static_assert (ns::_curNamespace, "BEGIN_NAMESPACE: namespace has not been declared");
^~~~~~~~~~~~~
prog.cc:27:5: note: in expansion of macro 'BEGIN_NAMESPACE'
BEGIN_NAMESPACE (whoops)
^~~~~~~~~~~~~~~
And more importantly this (and this is why we need the BEGIN_NAMESPACE macro):
DECLARE_NAMESPACE (Bar)
BEGIN_NAMESPACE (Bar)
DECLARE_NAMESPACE (BarWhoops)
BEGIN_NAMESPACE (Barwhoops)
END_NAMESPACE
END_NAMESPACE
Generates this:
prog.cc:12:24: error: '_curNamespace' is not a member of 'Bar::Barwhoops'
static_assert (ns::_curNamespace, "BEGIN_NAMESPACE: namespace has not been declared");
^~~~~~~~~~~~~
prog.cc:42:5: note: in expansion of macro 'BEGIN_NAMESPACE'
BEGIN_NAMESPACE (Barwhoops)
^~~~~~~~~~~~~~~
Which is just dandy.
So, you know, what's not to like?
Live demo - uncomment line 3 to see those compiler errors.
you can use a variable and change its value with 'NAMESPACE_BEGIN' and 'NAMESPACE_END'
the variable __name represent the current full namespace position
like "abc::def::detail"
std::string __name = "";
std::string & __append(std::string & str, const char * ptr) {
if (!str.empty()) {
str.append("::");
}
str.append(ptr);
return str;
}
std::string & __substr(std::string & str, const char * ptr) {
if (str.length() == strlen(ptr)) {
str.clear();
} else {
str = str.substr(0, str.length() - strlen(ptr) - 2);
}
return str;
}
#define NAMESPACE_NAME __name
#define CONCATENATE_DIRECT(s1, s2) s1##s2
#define CONCATENATE(s1, s2) CONCATENATE_DIRECT(s1, s2)
#ifdef _MSC_VER
# define ANONYMOUS_VARIABLE(str) CONCATENATE(str, __COUNTER__)
#else
# define ANONYMOUS_VARIABLE(str) CONCATENATE(str, __LINE__)
#endif
#define APPEND_NAME(x) std::string ANONYMOUS_VARIABLE(__start_name) = __append(__name, #x)
#define SUBSTR_NAME(x) std::string ANONYMOUS_VARIABLE(__end_name) = __substr(__name, #x)
#define NAMESPACE_BEGIN(x) \
namespace x { \
APPEND_NAME(x);
#define NAMESPACE_END(x) \
SUBSTR_NAME(x); \
}
then you can use the NAMESPACE_NAME macro for the full name or you can extract the last name from it
Here's a way. The core idea came from the line of thought:
Q: How can I define multiple things with the same name accessible from the same scope?
A: Make them all functions with different parameter types. And if all of them have identical bodies, it doesn't matter which one gets called.
Q: How can I generate an unlimited set of different parameter types?
A: A class template.
Q: How can I make sure a call to that set of overloaded functions will never be ambiguous?
A: Make sure the binary relation "is implicitly convertible from" is a complete ordering on the parameter types, and use a unique minimal element for the argument type.
#include <type_traits>
#include <functional>
struct NamespaceHandleObj {
template <const NamespaceHandleObj* Handle1, const NamespaceHandleObj* Handle2>
struct less : public std::bool_constant<std::less<>{}(Handle1, Handle2)> {};
};
template <>
struct NamespaceHandleObj::less<nullptr, nullptr> : public std::false_type {};
template <const NamespaceHandleObj* Handle1>
struct NamespaceHandleObj::less<Handle1, nullptr> : public std::false_type {};
template <const NamespaceHandleObj* Handle2>
struct NamespaceHandleObj::less<nullptr, Handle2> : public std::true_type {};
template <const NamespaceHandleObj* Handle>
struct NamespaceParamType
{
constexpr NamespaceParamType() noexcept = default;
template <const NamespaceHandleObj* Other,
typename = std::enable_if_t<NamespaceHandleObj::less<Other, Handle>::value>>
constexpr NamespaceParamType(NamespaceParamType<Other>) noexcept {}
};
#define NAMESPACE_UTILS_TOSTR1(x) #x
#define NAMESPACE_UTILS_TOSTR(x) NAMESPACE_UTILS_TOSTR1(x)
#define BEGIN_NAMESPACE(ns) \
namespace ns { \
namespace { \
constexpr NamespaceHandleObj namespace_handle_{}; \
constexpr const char* current_ns_(
NamespaceParamType<&namespace_handle_>) noexcept \
{ return NAMESPACE_UTILS_TOSTR(ns); } \
}
#define END_NAMESPACE }
#define CURRENT_NAMESPACE (current_ns_(NamespaceParamType<nullptr>{}))
The code above is C++17, but it wouldn't be hard to port it to previous versions, even all the way to C++03.
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;