errors about overload new operator in a template class - c++

I want to overload new operator in a template class. But something wrong happends.
In file test4.h, I defined a class
#include <stddef.h>
#include <iostream>
template <class T>
class lei{
public:
T me;
static void* operator new(size_t size);
};
test4.cpp implement the new operator.
#include "test4.h"
template <class T>
void* lei<T>::operator new(size_t size){
std::cout << size << std::endl;
}
main.cpp
#include "test4.h"
int main(){
lei<size_t> *pl;
pl = new lei<size_t>;
}
I compile the cpp files to .o file. Everything is OK.
But when I link them to the executable file, error occurs:
main.o: In function `main':
main.c:(.text+0x19): undefined reference to `lei<unsigned int>::operator new(unsigned int)'
collect2: ld returned 1 exit status
but everything is OK, if I don't use template.Why that happens?
So, I hope someone can help me.

You need to put the template implementation in the header file also. The implementation needs to be visible to the compiler when it needs to instantiate the template.
This the C++FAQ 35.12 to learn why this is necessary, and the following entries for ways to fix it.
(Also, your implementation of operator new should return something, it should not compile otherwise.)

Move the implementation of the operator to the header file, and it should compile.

The implementation of your new operator needs to be in the header file as well, not the .cpp file.

Related

Forward declaration is not working as expected

I'm defining a class SpatialCriterionCallback in a header file "spatialcriterion.h" like this:
#include "ros/ros.h"
#include "neuromorphic_stereo/MatchingCandidates.h"
#include <vector>
class SpatialCriterionCallback
{
public:
// constructors & destructors
SpatialCriterionCallback()=default;
SpatialCriterionCallback(ros::NodeHandle);
~SpatialCriterionCallback()=default;
private:
std::vector<neuromorphic_stereo::MatchingCandidates> matching_candidates;
void subscriberCallbackFunction(constneuromorphic_stereo::MatchingCandidates&);
}
Then in the file "spatialcriterion.cpp" I'm defining a constructor that invokes a ros::SubscriberNode like this:
#include "spatialcriterioncallback.h"
SpatialCriterionCallback::SpatialCriterionCallback(ros::NodeHandle n)
{
this->n =n;
this->time_criterion_topic_handle = this->n.subscribe("TimeCriterionTopic",
1e4,
&SpatialCriterionCallback::subscriberCallbackFunction,
this);
}
When I try compiling this within a qtcreator project, the compiler tells me
error: undefined reference to
`SpatialCriterionCallback::subscriberCallbackFunction(neuromorphic_stereo::MatchingCandidates_ const&)'
When I add the following lines to my "spatialcriterion.cpp" file it will compile just fine:
void SpatialCriterionCallback::subscriberCallbackFunction(const neuromorphic_stereo::MatchingCandidates & msg){
this->matching_candidates.push_back(msg);
}
Now my question is: Shouldn't this code compile without the function definition, because the function subscriberCallbackFunction() has already been declared in "spatialcriterion.h"? Why is it necessary for the compiler to have the function defined?
I also tried finding an explanation to this behaviour here, but all the other posts about failing forward declaration (like this or this) aren't exactly what I'm looking for.
Even if you declared the method in your class, it doesn't exist.
When you reference that function in the constructor, the linker tells you that it needs to know where the method is.
The code compiles fine, it doesn't link.

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)

Something wrong with linking

I have 3 C++ files:
genericStack.h:
template <class T>
class Stack{
public:
Stack (int size){
top = -1;
MAX_SIZE = size;
v = new T (size);
}
~Stack(){ delete v;}
T pop();
void push (T);
class Underflow{};
class Overflow{};
private:
int top;
int MAX_SIZE;
T* v;
};
genericStackImpl.c++:
#include "genericStack.h"
template <class T>
void Stack <T> :: push (T c){
if (top == MAX_SIZE - 1) throw Overflow();
v[++top] = c;
}
template <class T>
T Stack <T> :: pop(){
if (top < 0) throw Underflow();
return v[top--];
}
driver.c++:
#include <iostream>
#include "genericStack.h"
int main(){
Stack<char> sc(3);
try{
while (true) sc.push ('p');
}
catch (Stack<char>::Overflow){std::cout << "Overflow caught\n";}
try{
while (true) std::cout << sc.pop() << '\n';
}
catch (Stack<char>::Underflow){ std::cout << "Underflow caught\n";}
return 0;
}
When i compile using g++ 4.5:
g++ -o driver driver.c++ genericStackImpl.c++
I get these errors:
/tmp/ccLXRXgF.o: In function `main':
driver.c++:(.text+0x2e): undefined reference to `Stack<char>::push(char)'
driver.c++:(.text+0x3c): undefined reference to `Stack<char>::pop()'
collect2: ld returned 1 exit status
I dont understand what the problem is. If i move the implementation in the driver file, then it compiles and runs.
Generally speaking, template definitions need to also be in the header file. An exception to this is when you are explicitly instantiating, or using explicit specialisations, neither of which you are doing.
One way to solve this would be to move the contents of genericStackImpl.c++ to the bottom of its header file.
The reason for this is because template functions are not actual functions, they are just templates. A template is used (instantiated) to create actual functions, and those are what you link against.
There are no functions in genericStackImpl.c++. The functions only get created once you use them, i.e. the first time the compiler sees sc.push and sc.pop. Unfortunately, when driver.c++ tries to create these functions, it can't find the template bodies -- they hidden in genericStackImpl.c++! Instead, it just compiles a reference to those functions, hoping that some other file will make them.
Finally, when it comes to link time, the linker can't find the function anywhere, so it gives you an error.
Another way to solve this would be to explicitly instantiate those functions yourself in genericStackImpl.c++, i.e.
template class Stack<char>;
This will create the functions, and the linker will find them.
The problem with this approach is that it require you to know what types your going to be using in your stack beforehand, so most people just put template definitions in the header file.

C++ Template Class Problem

Hi everybody I just wanted to practice some c++ template but i get linker errors. Can anybody help me please?
Here is my code:
// File: MyClass.h
#ifndef _MYCLASS_H
#define _MYCLASS_H
template<class T> class MyClass {
T value;
public:
MyClass(T v);
~MyClass();
};
#endif // _MYCLASS_H
// File: MyClass.cpp
#include "MyClass.h"
template<class T> MyClass<T>::MyClass(T v) {
value = v;
}
template<class T> MyClass<T>::~MyClass() {
}
// File: main.cpp
#include "MyClass.h"
int main() {
MyClass<int> test(10);
return 0;
}
Here is command line output:
g++ main.cpp -c
g++ MyClass.cpp -c
g++ main.o MyClass.o -o Out
main.o: In function `main':
main.cpp:(.text+0x1a): undefined reference to `MyClass<int>::MyClass(int)'
main.cpp:(.text+0x2b): undefined reference to `MyClass<int>::~MyClass()'
collect2: ld returned 1 exit status
make: *** [all] Error 1
As you can see I'm using Ubuntu 10.04 and GNU C++ Compiler.
Am I missing something in this code?
Thanks for replies. It works but isn't there a better way to protect the code?
For example what if I want to create a non-opensource library?!
I want to export the code to a static library. and link the library to other projects ...
You have to put full template into the header. Compiler needs to see the body of the template methods at the site of template instantiation - main.cpp in your case. See, for example, C++ FAQ.
You should put template classes and inline methods into header files. You can't seperate definition and implementation in their case.
#Nikolai N Fetissov has the right solution. I would add to this that a nice way to do this, if you want to keep the implementation and templated function definitions separate is that you can put the implementations into MyClass.hxx and include it at the end of your MyClass.h
// File: MyClass.h
#ifndef _MYCLASS_H
#define _MYCLASS_H
template<class T> class MyClass
{
T value;
public:
MyClass(T v);
~MyClass();
};
#include "MyClass.hxx" /// <--- like this
#endif // _MYCLASS_H
It's important to remember what a template is. It is a template for generating code if needed; it is not code itself.
So declaring a template class and writing implementations for those methods does not generate any object code for that class; it simply provides a template for doing so if necessary.
When a template class is instantiated with an actual argument, the compiler will generate the code from the template class. In order to do that, it needs to be able to see the the templates. But since you've only #includeed the .h file, and the implementation of the methods in in the .cpp file, the compiler won't be able to generate the object code for the function implementations. Then, when the linker looks for those definitions it won't find it.
All of this is a long-winded way of getting to the same result the other answers did -- you need to put the implementation in the header file with the class declaration. But it may help to know why that is.

C++ template function compiles in header but not implementation

I'm trying to learn templates and I've run into this confounding error. I'm declaring some functions in a header file and I want to make a separate implementation file where the functions will be defined.
Here's the code that calls the header (dum.cpp):
#include <iostream>
#include <vector>
#include <string>
#include "dumper2.h"
int main() {
std::vector<int> v;
for (int i=0; i<10; i++) {
v.push_back(i);
}
test();
std::string s = ", ";
dumpVector(v,s);
}
Now, here's a working header file (dumper2.h):
#include <iostream>
#include <string>
#include <vector>
void test();
template <class T> void dumpVector(const std::vector<T>& v,std::string sep);
template <class T> void dumpVector(const std::vector<T>& v, std::string sep) {
typename std::vector<T>::iterator vi;
vi = v.cbegin();
std::cout << *vi;
vi++;
for (;vi<v.cend();vi++) {
std::cout << sep << *vi ;
}
std::cout << "\n";
return;
}
With implementation (dumper2.cpp):
#include <iostream>
#include "dumper2.h"
void test() {
std::cout << "!olleh dlrow\n";
}
The weird thing is that if I move the code that defines dumpVector from the .h to the .cpp file, I get the following error:
g++ -c dumper2.cpp -Wall -Wno-deprecated
g++ dum.cpp -o dum dumper2.o -Wall -Wno-deprecated
/tmp/ccKD2e3G.o: In function `main':
dum.cpp:(.text+0xce): undefined reference to `void dumpVector<int>(std::vector<int, std::allocator<int> >, std::basic_string<char, std::char_traits<char>, std::allocator<char> >)'
collect2: ld returned 1 exit status
make: *** [dum] Error 1
So why does it work one way and not the other? Clearly the compiler can find test(), so why can't it find dumpVector?
The problem you're having is that the compiler doesn't know which versions of your template to instantiate. When you move the implementation of your function to x.cpp it is in a different translation unit from main.cpp, and main.cpp can't link to a particular instantiation because it doesn't exist in that context. This is a well-known issue with C++ templates. There are a few solutions:
1) Just put the definitions directly in the .h file, as you were doing before. This has pros & cons, including solving the problem (pro), possibly making the code less readable & on some compilers harder to debug (con) and maybe increasing code bloat (con).
2) Put the implementation in x.cpp, and #include "x.cpp" from within x.h. If this seems funky and wrong, just keep in mind that #include does nothing more than read the specified file and compile it as if that file were part of x.cpp In other words, this does exactly what solution #1 does above, but it keeps them in seperate physical files. When doing this kind of thing, it is critical that you not try to compile the #included file on it's own. For this reason, I usually give these kinds of files an hpp extension to distinguish them from h files and from cpp files.
File: dumper2.h
#include <iostream>
#include <string>
#include <vector>
void test();
template <class T> void dumpVector( std::vector<T> v,std::string sep);
#include "dumper2.hpp"
File: dumper2.hpp
template <class T> void dumpVector(std::vector<T> v, std::string sep) {
typename std::vector<T>::iterator vi;
vi = v.begin();
std::cout << *vi;
vi++;
for (;vi<v.end();vi++) {
std::cout << sep << *vi ;
}
std::cout << "\n";
return;
}
3) Since the problem is that a particular instantiation of dumpVector is not known to the translation unit that is trying to use it, you can force a specific instantiation of it in the same translation unit as where the template is defined. Simply by adding this: template void dumpVector<int>(std::vector<int> v, std::string sep); ... to the file where the template is defined. Doing this, you no longer have to #include the hpp file from within the h file:
File: dumper2.h
#include <iostream>
#include <string>
#include <vector>
void test();
template <class T> void dumpVector( std::vector<T> v,std::string sep);
File: dumper2.cpp
template <class T> void dumpVector(std::vector<T> v, std::string sep) {
typename std::vector<T>::iterator vi;
vi = v.begin();
std::cout << *vi;
vi++;
for (;vi<v.end();vi++) {
std::cout << sep << *vi ;
}
std::cout << "\n";
return;
}
template void dumpVector<int>(std::vector<int> v, std::string sep);
By the way, and as a total aside, your template function is taking a vector by-value. You may not want to do this, and pass it by reference or pointer or, better yet, pass iterators instead to avoid making a temporary & copying the whole vector.
This was what the export keyword was supposed to accomplish (i.e., by exporting the template, you'd be able to put it in a source file instead of a header. Unfortunately, only one compiler (Comeau) ever really implemented export completely.
As to why the other compilers (including gcc) didn't implement it, the reason is pretty simple: because export is extremely difficult to implement correctly. Code inside the template can change meaning (almost) completely, based on the type over which the template is instantiated, so you can't generate a conventional object file of the result of compiling the template. Just for example, x+y might compile to native code like mov eax, x/add eax, y when instantiated over an int, but compile to a function call if instantiated over something like std::string that overloads operator+.
To support separate compilation of templates, you have to do what's called two-phase name lookup (i.e., lookup the name both in the context of the template and in the context where the template is being instantiated). You typically also have the compiler compile the template to some sort of database format that can hold instantiations of the template over an arbitrary collection of types. You then add in a stage between compiling and linking (though it can be built into the linker, if desired) that checks the database and if it doesn't contain code for the template instantiated over all the necessary types, re-invokes the compiler to instantiate it over the necessary types.
Due to the extreme effort, lack of implementation, etc., the committee has voted to remove export from the next version of the C++ standard. Two other, rather different, proposals (modules and concepts) have been made that would each provide at least part of what export was intended to do, but in ways that are (at least hoped to be) more useful and reasonable to implement.
Template parameters are resolved as compile time.
The compiler finds the .h, finds a matching definition for dumpVector, and stores it. The compiling is finished for this .h. Then, it continues parsing files and compiling files. When it reads the dumpVector implementation in the .cpp, it's compiling a totally different unit. Nothing is trying to instantiate the template in dumper2.cpp, so the template code is simply skipped. The compiler won't try every possible type for the template, hoping there will be something useful later for the linker.
Then, at link time, no implementation of dumpVector for the type int has been compiled, so the linker won't find any. Hence why you're seeing this error.
The export keyword is designed to solve this problem, unfortunately few compilers support it. So keep your implementation with the same file as your definition.
A template function is not real function. The compiler turns a template function into a real function when it encounters a use of that function. So the entire template declaration has to be in scope it finds the call to DumpVector, otherwise it can't generate the real function.
Amazingly, a lot of C++ intro books get this wrong.
This is exactly how templates work in C++, you must put the implementation in the header.
When you declare/define a template function, the compiler can't magically know which specific types you may wish to use the template with, so it can't generate code to put into a .o file like it could with a normal function. Instead, it relies on generating a specific instantiation for a type when it sees the use of that instantiation.
So when the implementation is in the .C file, the compiler basically says "hey, there are no users of this template, don't generate any code". When the template is in the header, the compiler is able to see the use in main and actually generate the appropriate template code.
Most compilers don't allow you to put template function definitions in a separate source file, even though this is technically allowed by the standard.
See also:
http://www.parashift.com/c++-faq-lite/templates.html#faq-35.12
http://www.parashift.com/c++-faq-lite/templates.html#faq-35.14