function is not found when instantiate a template function in VS2017 - c++

It might be a stupid question but I totally don't have any idea about the prompt.
The code is in a .cpp file
template <typename T> void foo2(T){}
template void foo2<int>(int);
VS2017 keeps telling me that: Function definition for function 'foo2' is not found. However, the code actually works in VS2017, no error message if I run it. I don't know if it is an IDE-specific problem or it is the code problem. As it is quite annoying, does anyone know why the prompt appears and how to fix it? Thanks!
=========Update===========
Here is the full code(Move the instantiation to .h file but still have the same problem):
test.h
#pragma once
template <typename T> void foo2(T);
template void foo2<int>(int);
test.cpp
#include "test.h"
#include "stdafx.h"
template <typename T>void foo2(T){}
main file
#include "stdafx.h"
#include "test.h"
int main()
{
int a = 1;
foo2(a);
}
I tend to believe that an IDE-specific question. If I ask VS to show the potential fix, it will create the following code in .cpp file:
template void foo2(int)
{
return template void();
}
which is definitely wrong. Even cannot pass the compilation.

With the expanded code, I get the idea.
You actually want extern template void foo2<int>(int); in the .h file. There should be only one instantiation, in test.cpp. extern template is new in C++11, so while your book might not yet cover it it's certainly understood by VS2017.

Related

Why separating template definition/declaration sometimes work

I'm wide awake at 1AM trying to figure out a compilation error I'm having.
I can't really write the exact code but I'll do my best to make my question clear.
ClassWithTemplateFunction.hpp
#include "StructA.hpp"
#include "ClassB.hpp"
class ClassWithTemplateFunction
{
template<typename A>
void MyTemplateFunc();
}
ClassWithTemplateFunction.cpp
template<typename T>
void ClassWithTemplateFunction::MyTemplateFunc()
{
// code block
}
StructA.hpp
struct ClassWithTemplateFunction;
struct StructA
{
void StructAFunc(ClassWithTemplateFunction* templ);
}
StructA.cpp
#include "ClassWithTemplateFunction.hpp"
#include "StructA.hpp"
StructA::StructAFunc(ClassWithTemplateFunction* templ)
{
templ->MyTemplateFunc<SomeTemplate>();
}
The above codes work great. "SomeTemplate" is also another class. But then I added a new class which also uses the template function:
ClassB.hpp
class ClassWithTemplateFunction;
class ClassB
{
void ClassBFunc();
}
ClassB.cpp
#include "ClassB.hpp"
#include "ClassWithTemplateFunction.hpp"
void ClassB::ClassBFunc(ClassWithTemplateFunction* templ)
{
templ->MyTemplateFunc<SomeTemplate>();
}
And for some reason, this now introduced a linker error (undefined reference to MyTemplateFunc()). I can't figure out why it suddenly produced this issue. Obviously it can be fixed by moving the definition to the header file, but I want to understand, before adding ClassB, the code works just fine.
If ClassWithTemplateFunction.cpp instantiates MyTemplateFunc with some template arguments somehow (e.g. by calling it), you'll be able to use MyTemplateFunc with the exact same arguments anywhere in the program.
That's why you don't get an undefined reference in the first case.
But it's not possible for an invocation of MyTemplateFunc with the exact same template arguments in a different file to cause an undefined reference. Please check if the template argument is really the same in both cases.

Issue with extern templates c++

I'm using C++ and I'm having struggle with extern templates. In opposite to C# the whole template implementation is really nasty in C++ :(
template_test.hpp
template<class T>
class CFoo {
public:
T Foo_Func(const T& test);
};
template_test.cpp
#include "Template_Test.hpp"
template<class T>
T CFoo<T>::Foo_Func(const T& test)
{
return test;
}
template_test2.hpp
#include "Template_Test.hpp"
extern template class CFoo<int>;
int Template_Tests();
template_test2.cpp
#include "Template_Test2.hpp"
int Template_Tests()
{
CFoo<int> foo_instance;
//this causes an undefined reference
int res = foo_instance.Foo_Func(1);
return res;
}
why does the linker not find my function. I thought extern templates worked the same why as extern variables.
(Put extern int test; in the header file and int test = 0 in the source file.)
thanks for your support:)
Solution 1
One way to solve this issue is to implements the template class's function without function's definitions. in this case:
template<class T>
class CFoo {
public:
T Foo_Func(const T& test) {
return test;
}
};
And then, you don't even need the extern part. I aware that your programmer sense keep telling you to avoid from this, and always to separate between your class functions' definitions, and their implementation- but in template case in c++, it's the easiest solution for this language's huge problem.
An important thing that you need to know- there is a big different between the solutions for this issue between differnt IDEs, but this easy solution works in most of them (if not always).
Solution 2
Another option, if you still want to separate the implementations from the definitions you can include the .cpp file, as well as the .hpp/.h file:
template_test2.hpp
#include "Template_Test.hpp"
#include "Template_Test.cpp"
/*extern template class CFoo<int>;*/ // Again, you don't need this extern
int Template_Tests();
Solution 3
It is the closest way to the way that you tried. in the end of template_test.cpp file, add the following line:
template class CFoo<int>;
and remove the line extern template class CFoo<int>; from the template_test2.hpp file.
I hope that you will find it helping, Korel.

CUDA C++ Templating of Kernel Parameter

I'm trying to templatize a CUDA kernel based on a boolean variable (as shown here: Should I unify two similar kernels with an 'if' statement, risking performance loss?), but I keep getting a compiler error that says my function is not a template. I think that I'm just missing something obvious so it's pretty frustrating.
The following does NOT work:
util.cuh
#include "kernels.cuh"
//Utility functions
kernels.cuh
#ifndef KERNELS
#define KERNELS
template<bool approx>
__global__ void kernel(...params...);
#endif
kernels.cu
template<bool approx>
__global__ void kernel(...params...)
{
if(approx)
{
//Approximate calculation
}
else
{
//Exact calculation
}
}
template __global__ void kernel<false>(...params...); //Error occurs here
main.cu
#include "kernels.cuh"
kernel<false><<<dimGrid,dimBlock>>>(...params...);
The following DOES work:
util.cuh
#include "kernels.cuh"
//Utility functions
kernels.cuh
#ifndef KERNELS
#define KERNELS
template<bool approx>
__global__ void kernel(...params...);
template<bool approx>
__global__ void kernel(...params...)
{
if(approx)
{
//Approximate calculation
}
else
{
//Exact calculation
}
}
#endif
main.cu
#include "kernels.cuh"
kernel<false><<<dimGrid,dimBlock>>>(...params...);
If I throw in the
template __global__ void kernel<false>(...params...);
line at the end of kernels.cuh it also works.
I get the following errors (both referring to the marked line above):
kernel is not a template
invalid explicit instantiation declaration
If it makes a difference I compile all of my .cu files in one line, like:
nvcc -O3 -arch=sm_21 -I. main.cu kernels.cu -o program
All explicit specialization declarations must be visible at the time of the template instantiation. Your explicit specialization declaration is visible only in the kernels.cu translation unit, but not in main.cu.
The following code is indeed working correctly (apart from adding a __global__ qualifier at the explicit instantiation instruction).
#include<cuda.h>
#include<cuda_runtime.h>
#include<stdio.h>
#include<conio.h>
template<bool approx>
__global__ void kernel()
{
if(approx)
{
printf("True branch\n");
}
else
{
printf("False branch\n");
}
}
template __global__ void kernel<false>();
int main(void) {
kernel<false><<<1,1>>>();
getch();
return 0;
}
EDIT
In C++, templated functions are not compiled until an explicit instantiation of the function is encountered. From this point of view, CUDA, which now fully supports templates, behaves exactly the same way as C++.
To make a concrete example, when the compiler finds something like
template<class T>
__global__ void kernel(...params...)
{
...
T a;
...
}
it just checks the function syntax, but produces no object code. So, if you would compile a file with a single templated function as above, you will have an "empty" object file. This is reasonable, since the compiler would not know which type assigning to a.
The compiler produces an object code only when it encounters an explicit instantiation of the function template. This is, at that moment, how compilation of templated functions work and this behavior introduces a restriction for multiple-file projects: the implementation (definition) of a templated function must be in the same file as its declaration. So, you cannot separate the interface contained in kernels.cuh in a header file separated from kernels.cu, which is the main reason why the first version of your code does not compile. Accordingly, you must include both interface and implementation in any file that uses the templates, namely, you must include in main.cu both, kernels.cuh and kernels.cu.
Since no code is generated without an explicit instantiation, compilers tolerate the inclusion more than once of the same template file with both declarations and definitions in a project without generating linkage errors.
There are several tutorials on using templates in C++. An Idiot's Guide to C++ Templates - Part 1, apart from the irritating title, will provide you with a step-by-step introduction to the topic.

Multiple Template Instantiation

I'm designing a CUDA-C++ library with template classes. There are template functions my classes use, and they are invisible to main as well as the user. I need to specialize them explicitly because of the two steps of compiling to be performed, otherwise I'd get an "unresolved external" error when linking. Being this classes used in main.cpp, there's no way (I guess...) to tell nvcc what types are going to be used in tha main program, so I thought of using some macros to specialize them. Here's a simplified versione of the code:
//CUDA_functions.h
// CUDA functions declared here and included in files that will be compiled
// with g++. Those functions are implemented in .cu files, compiled with nvcc
template <typename T>
void foo1(T x);
template <typename T>
void foo2(T x);
template <typename T>
void foo3(T x);
//fileA.h - included in main.cpp
#include "CUDA_functions.h"
template <typename T>
class A {
// it uses foo1 & foo2 inside
}
//fileB.h - included in main.cpp
#include "CUDA_functions.h"
template <typename T>
class B {
// it uses foo1 & foo3 inside
}
//macros.h
#define _USE_CLASS_A(T) template void foo1(T); \
template void foo2(T); /**/
#define _USE_CLASS_B(T) template void foo1(T); \
template void foo3(T); /**/
//user_spec.cu - template specializations by user. This is the first file to be
// - compiled and it doesn't know what classes are going to be used
// say, user wants to use classes A & B: HERE THE ERROR RAISES!
#include "macros.h"
_USE_CLASS_A( int );
_USE_CLASS_B( int );
When I compile this code with Visual Studio, I get a warning about the double explicit instantiation (foo1), but when I compile it with g++ warning becomes an error!
I can't write macros like
#define _USE_FOO1(T) template void foo1(T) /**/
#define _USE_FOO2(T) template void foo2(T) /**/
#define _USE_FOO3(T) template void foo3(T) /**/
because the user doesn't have to worry about the existence of those functions and I'd like to specialize a list of them based on what class he/she is going to use. Last but not least, I found nothing about a "conditional specialization" of template. What can I do to solve? Thanks to everyone would be so nice to answer. Bye.
Is it for host code or device code? I believe CUDA does not support linking for device code. Linking template functions in host code has always been a bit fishy, CUDA or no CUDA.
Instead of having your hands dirty with macros -- how about putting them in a header, inside of namespace detail?
By convention, detail namespace indicates library internal stuff that you shouldn't ever access as a user.

Separate compiling with MinGW

Using this tutorial Makefile I want to build a simple program with a separate compiling, The main problem is that the IDE Eclpse Indigo C\C++ (prespective) or MinGW I cannot compile the files. The error which I get is :
undefined reference to double getAverage<int, 85u>(int (&) [85u])'
undefined reference to int getMax<int, 85u>(int (&) [85u])'
undefined reference to int getMin<int, 85u>(int (&) [85u])'
undefined reference to void print<int, 85u>(int (&) [85u])'
undefined reference to void sort<int, 85u>(int (&) [85u])'
undefined reference to void print<int, 85u>(int (&) [85u])'
The main.cpp file is this :
#include "Tools.h"
#include <iostream>
using namespace std;
int main(int argc,char* argv[])
{
int numbers[] = {1,-2,7,14,5,6,16,8,-2,7,14,5,6,16,8,-2,7,14,5,6,16,8,-2,7,14,5,6,16,8,-2,7,14,5,6,16,8,-2,7,14,5,6,16,8,-2,7,14,5,6,16,8,-2,7,14,5,6,16,8,-2,7,14,5,6,16,8,-2,7,14,5,6,16,8,-2,7,14,5,6,16,8,-2,7,14,5,6,16,8};
cout <<"Average = "<< getAverage(numbers) << endl;
cout <<"Max element = "<< getMax(numbers) << endl;
cout <<"Minimal element = "<< getMin(numbers) << endl;
print(numbers);
sort(numbers);
print(numbers);
return 0;
}
and I have a Tools.h file :
#ifndef TOOLS_H_
#define TOOLS_H_
#include <iostream>
int getBigger(int numberOne,int numberTwo);
template <typename T,size_t N> double getAverage(T (&numbers)[N]);
template <typename T,size_t N> T getMax(T (&numbers)[N]);
template <typename T,size_t N> T getMin(T (&numbers)[N]);
/**
* Implementing a simple sort method of big arrays
*/
template <typename T,size_t N> void sort(T (&numbers)[N]);
/**
* Implementing a method for printing arrays
*/
template <typename T,size_t N> void print(T (&numbers)[N]);
#endif
When you compile Tools.cpp your compiler has no idea about the template parameters that you have used in main.cpp. Therefore it compiles nothing related to this templates.
You need to include theses template definitions from the compilation unit that uses them. The file Tools.cpp is often renamed to something like Tools.inl to indicate that it's neither a header file nor a separate compilation unit.
The compilation unit "main.cpp" could look like this:
#include "tools.h"
#include "tools.inl"
main()
{
int number[] = {1,2,3};
getaverage(numbers);
}
Since the compiler identifies the required specialization it can generate the code from the implementation file.
For most cases, harper's answer is appropriate. But for completeness' sake, explicit template instantiation should also be mentioned.
When you include the implementation in every compilation unit, your template classes and functions will be instantiated and compiled in all of them. Sometimes, this is not desirable. It is mostly due to compile-time memory restrictions and compilation time, if your template classes and functions are very complicated. This becomes a very real issue when you, or the libraries you use rely heavily on template metaprogramming. Another situation could be that your template function implementations might be used in many compilation units, and when you change the implementation, you will be forced to re-compile all those compilation units.
So, the solution in these situations is to include a header file like your tools.h, and have a tools.cpp, implementing the templates. The catch is that, you should explicitly instantiate your templates for all the template arguments that will be used throughout your program. This is accomplished via adding the following to tools.cpp:
template double getAverage<int,85>(int (&numbers)[85]);
Note: You obviously have to do something about that "85", such as defining it in a header file and using it across tools.cpp and main.cpp
I've found this article which is useful : templates and header files
I declared the function in the Tools.h file and include there the file Tool.hpp and after this I defined them in the Tools.hpp file.
I haven't tried to compile .cpp and .c files together but maybe my example will help.
I had similar problem compiling two separate assembly files .s on mingw with standard gcc
compiler and i achieved it as follows:
gcc -m32 -o test test.s hello.s
-m32 means i'm compiling 32bit code
-o is the output file ( which in my example is the "test" file )
test.s and hello.s are my source files. test.s is the main file and hello.s has the helper function. (Oh, to mention is the fact that both files are in the same directory)