Putting all const values of a class in the same place - c++

I am trying to use some compile-time const values for a class and I am using constexpr/const, but I dislike a lot the fact that some will be inited in the hpp and some in the .cpp.
I've read the explanation related to string incompatibility with constexpr, but
isn't a trick to put all of them in the same location?
It's strange that in the modern C++ 11/14 you have to declare them in two separate locations.:)
Thank you
// test.hpp
class Test
{
// initialization values are located clear in the header
static constexpr int a_{10};
static constexpr int b_{20};
// we must go to the cpp file to see this string value
static const std::string str_;
};
// test.cpp
const std::string Test::str_{"abc"};

Related

How to use info passed as parameter to build a string defined in other CPP file

I have a C++ program that has several .hpp files with declarations of variables (most of them paths to a NFS filesystem) and .cpp files with the definitions of those variables.
In some of those variables, whose type is std::string, I need to build its content by appending some content passed as parameter in the main program. For instance:
File constants.hpp:
namespace constants {
extern std::string cudnn_version;
extern const std::string path_caffe_cuda;
extern const std::string path_caffe_cuda_cudnn;
}
File constants.cpp:
const std::string constants::path_caffe_cuda = "/nfs/apps/caffe/cuda";
const std::string constants::path_caffe_cuda_cudnn = constants::path_caffe_cuda + "/cudnn" + constants::cudnn_version;
The content of constants::cudnn_version is asked to the user in the main program as parameter and updated there. The problem is, when constants::path_caffe_cuda_cudnn variable must be built with the content of constants::cudnn_version variable, its content is still empty, so in some way the variable path_caffe_cuda_cudnn is evaluated before the constants::cudnn_version has the content passed by user.
How do you think I could fix the issue?
Thank you very much to everybody.
It could easily be done by using a function instead:
namespace constants {
extern std::string cudnn_version;
extern const std::string path_caffe_cuda;
inline std::string path_caffe_cuda_cudnn()
{
return constants::path_caffe_cuda + "/cudnn" + constants::cudnn_version;
}
}
As long as path_caffe_cuda_cudnn is not called until constants::cudnn_version have been initialized, then it will be okay.

Why the singleton initialization failed (link error) [duplicate]

Very simply put:
I have a class that consists mostly of static public members, so I can group similar functions together that still have to be called from other classes/functions.
Anyway, I have defined two static unsigned char variables in my class public scope, when I try to modify these values in the same class' constructor, I am getting an "unresolved external symbol" error at compilation.
class test
{
public:
static unsigned char X;
static unsigned char Y;
...
test();
};
test::test()
{
X = 1;
Y = 2;
}
I'm new to C++ so go easy on me. Why can't I do this?
If you are using C++ 17 you can just use the inline specifier (see https://stackoverflow.com/a/11711082/55721)
If using older versions of the C++ standard, you must add the definitions to match your declarations of X and Y
unsigned char test::X;
unsigned char test::Y;
somewhere. You might want to also initialize a static member
unsigned char test::X = 4;
and again, you do that in the definition (usually in a CXX file) not in the declaration (which is often in a .H file)
Static data members declarations in the class declaration are not definition of them.
To define them you should do this in the .CPP file to avoid duplicated symbols.
The only data you can declare and define is integral static constants.
(Values of enums can be used as constant values as well)
You might want to rewrite your code as:
class test {
public:
const static unsigned char X = 1;
const static unsigned char Y = 2;
...
test();
};
test::test() {
}
If you want to have ability to modify you static variables (in other words when it is inappropriate to declare them as const), you can separate you code between .H and .CPP in the following way:
.H :
class test {
public:
static unsigned char X;
static unsigned char Y;
...
test();
};
.CPP :
unsigned char test::X = 1;
unsigned char test::Y = 2;
test::test()
{
// constructor is empty.
// We don't initialize static data member here,
// because static data initialization will happen on every constructor call.
}
in my case, I declared one static variable in .h file, like
//myClass.h
class myClass
{
static int m_nMyVar;
static void myFunc();
}
and in myClass.cpp, I tried to use this m_nMyVar. It got LINK error like:
error LNK2001: unresolved external symbol "public: static class...
The link error related cpp file looks like:
//myClass.cpp
void myClass::myFunc()
{
myClass::m_nMyVar = 123; //I tried to use this m_nMyVar here and got link error
}
So I add below code on the top of myClass.cpp
//myClass.cpp
int myClass::m_nMyVar; //it seems redefine m_nMyVar, but it works well
void myClass::myFunc()
{
myClass::m_nMyVar = 123; //I tried to use this m_nMyVar here and got link error
}
then LNK2001 is gone.
Since this is the first SO thread that seemed to come up for me when searching for "unresolved externals with static const members" in general, I'll leave another hint to solve one problem with unresolved externals here:
For me, the thing that I forgot was to mark my class definition __declspec(dllexport), and when called from another class (outside that class's dll's boundaries), I of course got the my unresolved external error.
Still, easy to forget when you're changing an internal helper class to a one accessible from elsewhere, so if you're working in a dynamically linked project, you might as well check that, too.
When we declare a static variable in a class, it is shared by all the objects of that class. As static variables are initialized only once they are never initialized by a constructor. Instead, the static variable should be explicitly initialized outside the class only once using the scope resolution operator (::).
In the below example, static variable counter is a member of the class Demo. Note how it is initialized explicitly outside the class with the initial value = 0.
#include <iostream>
#include <string>
using namespace std;
class Demo{
int var;
static int counter;
public:
Demo(int var):var(var){
cout<<"Counter = "<<counter<<endl;
counter++;
}
};
int Demo::counter = 0; //static variable initialisation
int main()
{
Demo d(2), d1(10),d3(1);
}
Output:
Count = 0
Count = 1
Count = 2
In my case, I was using wrong linking.
It was managed c++ (cli) but with native exporting. I have added to linker -> input -> assembly link resource the dll of the library from which the function is exported. But native c++ linking requires .lib file to "see" implementations in cpp correctly, so for me helped to add the .lib file to linker -> input -> additional dependencies.
[Usually managed code does not use dll export and import, it uses references, but that was unique situation.]

Class declaration in a header file and static variables

Noob question, but would like to understand the following:
Imagine I have a multifile project. I'm specifying a class in a header file to be shared among all the files in the project, and I write this : static int test = 0; and in the next line this: static const int MAX = 4;
The first one would be an error trying to compile because of the one definition rule. But the second one will compile without errors. Why?
From what I understand, both have the same properties: whole execution storage duration, class scope and no linkage.
Any help?
EDIT: testing an external constant declaration in a header: extern const int MAX = 4; to force external linkage produced the expected error. So I don't understand why with the variable it gives me the error and with the constant it doesn't.
Try
static const int test = 0;
I've sometimes noticed compiler errors with the immediate initialization of static const variables in the header file. You can always use the declaration in the header
class MyClass
{
// ...
static const int test;
// ...
}
and initialize it in the corresponding .cpp file
const int MyClass::test = 0;
This should work properly with any other types than int as well.
Integer constants in C++ don't actually occupy any space in the object and don't act like variables in general. Think about them more like numbers that are given names in this particular context.

static ints, compilation units and the ternary operator

// SomeCls.h
class SomeCls
{
static const int PERIOD_ALARM_NORMAL = 5;
static const int PERIOD_ALARM_THRESH = 1;
void method()
{
bool b = true;
const int d = b ? PERIOD_ALARM_THRESH : PERIOD_ALARM_NORMAL;
}
} obj;
It is going to build ok. Now take out the method() implementation and place it in a cpp file:
//SomeCls.cpp
#include "SomeCls.h"
void SomeCls::method()
{
bool b = true;
const int d = b ? PERIOD_ALARM_THRESH : PERIOD_ALARM_NORMAL;
}
Why does mr. linker say
undefined reference to SomeCls::PERIOD_ALARM_NORMAL' undefined
reference toSomeCls::PERIOD_ALARM_THRESH'
?
Thanks
EDIT:
It seems to me that that inside .h, the ternary operator takes static const ints it as rvalues but ... outside the decalrative .h, it regards them as lvalue and needs definition.
This is what I have managed to understand from the answers below. Kudos to Bada compiler (some eabi linux thinggie)
If the compiler can't see all the static class constants' values, then you have to provide definitions for them so that they'll actually be stored somewhere. Add the following to your cpp file:
const int SomeCls::PERIOD_ALARM_NORMAL;
const int SomeCls::PERIOD_ALARM_THRESH;
This is a GCC limitation, but it's completely standard comforming. Technically a static const int is still an lvalue. You've provided the value inline so compiler will almost always use it as an rvalue. There is one exception. The abstract instructions emitted by the compiler for ternary operators queries the address of lvalues. Hence the error you're seeing.
You can work around this by using enum instead. Or if you're using a new version of GCC constexpr was added to standard to fix this exact problem (named and typed rvalues).
Alternatively you can provide the linker with a definition for the constants. E.g. in your classes cpp file add the a line like
// I wish I had constexpr
const int SomeCls::PERIOD_ALARM_NORMAL;
const int SomeCls::PERIOD_ALARM_THRESH;
As a side note: I was a staunch proponent of static const for class scope constants. Then I found out that MSVC doesn't allow for static const float with the value inline. So the only values you can portably put in a static const are integers, in which case enums provide all the same features plus the guarantee that they'll never silently convert to an lvalue.
If, for whatever reason, you compiler simply refuses to link the code (like GCC 4.4.5 does), here's a simple fix: Replace the static const ints with an enum.
// someclass.h
// include guards, blabla
class SomeClass
{
enum AlarmPeriod{
PERIOD_ALARM_NORMAL = 5,
PERIOD_ALARM_THRESH = 1
};
public:
void method();
};
// someclass.cpp
#include "someclass.h"
void SomeClass::method(){
bool b = true;
const int d = b ? PERIOD_ALARM_THRESH : PERIOD_ALARM_NORMAL;
}
// main.cpp
#include "someclass.h"
int main(){
someclass sc;
sc.method();
}
This links cleanly with GCC 4.4.5, which wouldn't link the former version, even though both are technically the same.
Note that you cannot, amongst other things, take the address of PERIOD_ALARM_NORMAL and PERIOD_ALARM_TRESH anymore, because both names are just aliases for their respective values.

How do I define string constants in C++? [duplicate]

This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
C++ static constant string (class member)
static const C++ class member initialized gives a duplicate symbol error when linking
My experience with C++ pre-dated the addition of the string class, so I'm starting over in some ways.
I'm defining my header file for my class and want to create a static constant for a url. I'm attempting this by doing as follows:
#include <string>
class MainController{
private:
static const std::string SOME_URL;
}
const std::string MainController::SOME_URL = "www.google.com";
But this give me a duplicate definition during link.
How can I accomplish this?
Move the
const std::string MainController::SOME_URL = "www.google.com";
to a cpp file. If you have it in a header, then every .cpp that includes it will have a copy and you will get the duplicate symbol error during the link.
You need to put the line
const std::string MainController::SOME_URL = "www.google.com";
in the cpp file, not the header, because of the one-definition rule. And the fact that you cannot directly initialize it in the class is because std::string is not an integral type (like int).
Alternatively, depending on your use case, you might consider not making a static member but using an anonymous namespace instead. See this post for pro/cons.
Define the class in the header file:
//file.h
class MainController{
private:
static const std::string SOME_URL;
}
And then, in source file:
//file.cpp
#include "file.h"
const std::string MainController::SOME_URL = "www.google.com";
You should put the const std::string MainController::SOME_URL = "www.google.com"; definition into a single source file, not in the header.