I have written a C++ STL like bitset class:
template<size_t N>
class bitset {
public:
...........
friend std::ostream& operator << (std::ostream &, bitset<N> const&);
private:
..........
};
// end of class
template<size_t N>
std::ostream& operator << (std::ostream &os, bitset<N> const& rhs) {
............
.........
return os;
}
And I am trying to use it like this:
bitset<5> foo; // success
std::cout << foo << std::endl; // fail
And the error message is -
undefined reference to `operator<<(std::ostream&, bitSet<5u> const&)
What's the problem actually?
Your friend's declaration must be a template as well, just like the definition is:
template <size_t N>
class bitset {
public:
template <size_t M>
friend std::ostream& operator << (std::ostream &, bitset<M> const&);
};
template <size_t M>
std::ostream& operator << (std::ostream &os, bitset<M> const& rhs) {
return os;
}
Alternatively, you could declare the operator<< directly within the class scope:
template<size_t N>
class bitset {
public:
friend std::ostream& operator << (std::ostream & os, bitset const&) {
return os;
}
};
Some possible answers to your question are given here.
In addition to the answer of Piotr S., you can also predeclare the function templates:
template<size_t N> class bitset;
template<size_t N> std::ostream& operator << (std::ostream &, bitset<N> const&);
//now comes your class definition
Related
I am reading Section 2.4 of the book "C++ tempalte, a complete guide".
I tried to override output operator (<<) as a function template outside the class template Stack<>.
Below is my code, but it doesn't work.
#include <iostream>
#include <string>
#include <vector>
template<class T>
class Stack
{
private:
std::vector<T> v;
public:
void push(T a);
void printOn(std::ostream & os) const;
template <typename U>
friend std::ostream& operator<< (std::ostream& out, const Stack<U> & s);
};
template<typename T>
std::ostream& operator<< (std::ostream out, const Stack<T> & s)
{
s.printOn(out);
return out;
}
template<class T>
void Stack<T>::push(T a)
{
v.push_back(a);
}
template<class T>
void Stack<T>::printOn(std::ostream & out) const
{
for(T const & vi : v)
{out << vi << " ";}
}
int main()
{
Stack<int> s1;
s1.push(12);
s1.push(34);
std::cout << s1;
}
You are just omitting the &, which makes the operator<< inside and outside the class have different function signatures, they are both valid for std::cout << s1, hence the ambiguity
template<typename T>
std::ostream& operator<< (std::ostream& out, const Stack<T> & s)
// ^
{
s.printOn(out);
return out;
}
I have read couple of the questions regarding my problem on StackOverflow.com now, and none of it seems to solve my problem. Or I maybe have done it wrong...
The overloaded << works if I make it into an inline function. But how do I make it work in my case?
warning: friend declaration std::ostream& operator<<(std::ostream&, const D<classT>&)' declares a non-template function
warning: (if this is not what you intended, make sure the function template has already been declared and add <> after the function name here) -Wno-non-template-friend disables this warning
/tmp/cc6VTWdv.o:uppgift4.cc:(.text+0x180): undefined reference to operator<<(std::basic_ostream<char, std::char_traits<char> >&, D<int> const&)'
collect2: ld returned 1 exit status
The code:
template <class T>
T my_max(T a, T b)
{
if(a > b)
return a;
else
return b;
}
template <class classT>
class D
{
public:
D(classT in)
: d(in) {};
bool operator>(const D& rhs) const;
classT operator=(const D<classT>& rhs);
friend ostream& operator<< (ostream & os, const D<classT>& rhs);
private:
classT d;
};
int main()
{
int i1 = 1;
int i2 = 2;
D<int> d1(i1);
D<int> d2(i2);
cout << my_max(d1,d2) << endl;
return 0;
}
template <class classT>
ostream& operator<<(ostream &os, const D<classT>& rhs)
{
os << rhs.d;
return os;
}
This is one of those frequently asked questions that have different approaches that are similar but not really the same. The three approaches differ in who you are declaring to be a friend of your function --and then on how you implement it.
The extrovert
Declare all instantiations of the template as friends. This is what you have accepted as answer, and also what most of the other answers propose. In this approach you are needlessly opening your particular instantiation D<T> by declaring friends all operator<< instantiations. That is, std::ostream& operator<<( std::ostream &, const D<int>& ) has access to all internals of D<double>.
template <typename T>
class Test {
template <typename U> // all instantiations of this template are my friends
friend std::ostream& operator<<( std::ostream&, const Test<U>& );
};
template <typename T>
std::ostream& operator<<( std::ostream& o, const Test<T>& ) {
// Can access all Test<int>, Test<double>... regardless of what T is
}
The introverts
Only declare a particular instantiation of the insertion operator as a friend. D<int> may like the insertion operator when applied to itself, but it does not want anything to do with std::ostream& operator<<( std::ostream&, const D<double>& ).
This can be done in two ways, the simple way being as #Emery Berger proposed, which is inlining the operator --which is also a good idea for other reasons:
template <typename T>
class Test {
friend std::ostream& operator<<( std::ostream& o, const Test& t ) {
// can access the enclosing Test. If T is int, it cannot access Test<double>
}
};
In this first version, you are not creating a templated operator<<, but rather a non-templated function for each instantiation of the Test template. Again, the difference is subtle but this is basically equivalent to manually adding: std::ostream& operator<<( std::ostream&, const Test<int>& ) when you instantiate Test<int>, and another similar overload when you instantiate Test with double, or with any other type.
The third version is more cumbersome. Without inlining the code, and with the use of a template, you can declare a single instantiation of the template a friend of your class, without opening yourself to all other instantiations:
// Forward declare both templates:
template <typename T> class Test;
template <typename T> std::ostream& operator<<( std::ostream&, const Test<T>& );
// Declare the actual templates:
template <typename T>
class Test {
friend std::ostream& operator<< <T>( std::ostream&, const Test<T>& );
};
// Implement the operator
template <typename T>
std::ostream& operator<<( std::ostream& o, const Test<T>& t ) {
// Can only access Test<T> for the same T as is instantiating, that is:
// if T is int, this template cannot access Test<double>, Test<char> ...
}
Taking advantage of the extrovert
The subtle difference between this third option and the first is in how much you are opening to other classes. An example of abuse in the extrovert version would be someone that wants to get access into your internals and does this:
namespace hacker {
struct unique {}; // Create a new unique type to avoid breaking ODR
template <>
std::ostream& operator<< <unique>( std::ostream&, const Test<unique>& )
{
// if Test<T> is an extrovert, I can access and modify *any* Test<T>!!!
// if Test<T> is an introvert, then I can only mess up with Test<unique>
// which is just not so much fun...
}
}
You can't declare a friend like that, you need to specify a different template type for it.
template <typename SclassT>
friend ostream& operator<< (ostream & os, const D<SclassT>& rhs);
note SclassT so that it doesn't shadow classT. When defining
template <typename SclassT>
ostream& operator<< (ostream & os, const D<SclassT>& rhs)
{
// body..
}
This worked for me without any compiler warnings.
#include <iostream>
using namespace std;
template <class T>
T my_max(T a, T b)
{
if(a > b)
return a;
else
return b;
}
template <class classT>
class D
{
public:
D(classT in)
: d(in) {};
bool operator>(const D& rhs) const {
return (d > rhs.d);
}
classT operator=(const D<classT>& rhs);
friend ostream& operator<< (ostream & os, const D& rhs) {
os << rhs.d;
return os;
}
private:
classT d;
};
int main()
{
int i1 = 1;
int i2 = 2;
D<int> d1(i1);
D<int> d2(i2);
cout << my_max(d1,d2) << endl;
return 0;
}
I think you shouldn't make friend in the first place.
You can create a public method call print, something like this (for a non template class):
std::ostream& MyClass::print(std::ostream& os) const
{
os << "Private One" << privateOne_ << endl;
os << "Private Two" << privateTwo_ << endl;
os.flush();
return os;
}
and then, outside the class (but in the same namespace)
std::ostream& operator<<(std::ostream& os, const MyClass& myClass)
{
return myClass.print(os);
}
I think it should work also for template class, but I haven't tested yet.
Here you go:
#include <cstdlib>
#include <iostream>
using namespace std;
template <class T>
T my_max(T a, T b)
{
if(a > b)
return a;
else
return b;
}
template <class classT>
class D
{
public:
D(classT in)
: d(in) {};
bool operator>(const D& rhs) const { return d > rhs.d;};
classT operator=(const D<classT>& rhs);
template<class classT> friend ostream& operator<< (ostream & os, const D<classT>& rhs);
private:
classT d;
};
template<class classT> ostream& operator<<(ostream& os, class D<typename classT> const& rhs)
{
os << rhs.d;
return os;
}
int main()
{
int i1 = 1;
int i2 = 2;
D<int> d1(i1);
D<int> d2(i2);
cout << my_max(d1,d2) << endl;
return 0;
}
I have read couple of the questions regarding my problem on StackOverflow.com now, and none of it seems to solve my problem. Or I maybe have done it wrong...
The overloaded << works if I make it into an inline function. But how do I make it work in my case?
warning: friend declaration std::ostream& operator<<(std::ostream&, const D<classT>&)' declares a non-template function
warning: (if this is not what you intended, make sure the function template has already been declared and add <> after the function name here) -Wno-non-template-friend disables this warning
/tmp/cc6VTWdv.o:uppgift4.cc:(.text+0x180): undefined reference to operator<<(std::basic_ostream<char, std::char_traits<char> >&, D<int> const&)'
collect2: ld returned 1 exit status
The code:
template <class T>
T my_max(T a, T b)
{
if(a > b)
return a;
else
return b;
}
template <class classT>
class D
{
public:
D(classT in)
: d(in) {};
bool operator>(const D& rhs) const;
classT operator=(const D<classT>& rhs);
friend ostream& operator<< (ostream & os, const D<classT>& rhs);
private:
classT d;
};
int main()
{
int i1 = 1;
int i2 = 2;
D<int> d1(i1);
D<int> d2(i2);
cout << my_max(d1,d2) << endl;
return 0;
}
template <class classT>
ostream& operator<<(ostream &os, const D<classT>& rhs)
{
os << rhs.d;
return os;
}
This is one of those frequently asked questions that have different approaches that are similar but not really the same. The three approaches differ in who you are declaring to be a friend of your function --and then on how you implement it.
The extrovert
Declare all instantiations of the template as friends. This is what you have accepted as answer, and also what most of the other answers propose. In this approach you are needlessly opening your particular instantiation D<T> by declaring friends all operator<< instantiations. That is, std::ostream& operator<<( std::ostream &, const D<int>& ) has access to all internals of D<double>.
template <typename T>
class Test {
template <typename U> // all instantiations of this template are my friends
friend std::ostream& operator<<( std::ostream&, const Test<U>& );
};
template <typename T>
std::ostream& operator<<( std::ostream& o, const Test<T>& ) {
// Can access all Test<int>, Test<double>... regardless of what T is
}
The introverts
Only declare a particular instantiation of the insertion operator as a friend. D<int> may like the insertion operator when applied to itself, but it does not want anything to do with std::ostream& operator<<( std::ostream&, const D<double>& ).
This can be done in two ways, the simple way being as #Emery Berger proposed, which is inlining the operator --which is also a good idea for other reasons:
template <typename T>
class Test {
friend std::ostream& operator<<( std::ostream& o, const Test& t ) {
// can access the enclosing Test. If T is int, it cannot access Test<double>
}
};
In this first version, you are not creating a templated operator<<, but rather a non-templated function for each instantiation of the Test template. Again, the difference is subtle but this is basically equivalent to manually adding: std::ostream& operator<<( std::ostream&, const Test<int>& ) when you instantiate Test<int>, and another similar overload when you instantiate Test with double, or with any other type.
The third version is more cumbersome. Without inlining the code, and with the use of a template, you can declare a single instantiation of the template a friend of your class, without opening yourself to all other instantiations:
// Forward declare both templates:
template <typename T> class Test;
template <typename T> std::ostream& operator<<( std::ostream&, const Test<T>& );
// Declare the actual templates:
template <typename T>
class Test {
friend std::ostream& operator<< <T>( std::ostream&, const Test<T>& );
};
// Implement the operator
template <typename T>
std::ostream& operator<<( std::ostream& o, const Test<T>& t ) {
// Can only access Test<T> for the same T as is instantiating, that is:
// if T is int, this template cannot access Test<double>, Test<char> ...
}
Taking advantage of the extrovert
The subtle difference between this third option and the first is in how much you are opening to other classes. An example of abuse in the extrovert version would be someone that wants to get access into your internals and does this:
namespace hacker {
struct unique {}; // Create a new unique type to avoid breaking ODR
template <>
std::ostream& operator<< <unique>( std::ostream&, const Test<unique>& )
{
// if Test<T> is an extrovert, I can access and modify *any* Test<T>!!!
// if Test<T> is an introvert, then I can only mess up with Test<unique>
// which is just not so much fun...
}
}
You can't declare a friend like that, you need to specify a different template type for it.
template <typename SclassT>
friend ostream& operator<< (ostream & os, const D<SclassT>& rhs);
note SclassT so that it doesn't shadow classT. When defining
template <typename SclassT>
ostream& operator<< (ostream & os, const D<SclassT>& rhs)
{
// body..
}
This worked for me without any compiler warnings.
#include <iostream>
using namespace std;
template <class T>
T my_max(T a, T b)
{
if(a > b)
return a;
else
return b;
}
template <class classT>
class D
{
public:
D(classT in)
: d(in) {};
bool operator>(const D& rhs) const {
return (d > rhs.d);
}
classT operator=(const D<classT>& rhs);
friend ostream& operator<< (ostream & os, const D& rhs) {
os << rhs.d;
return os;
}
private:
classT d;
};
int main()
{
int i1 = 1;
int i2 = 2;
D<int> d1(i1);
D<int> d2(i2);
cout << my_max(d1,d2) << endl;
return 0;
}
I think you shouldn't make friend in the first place.
You can create a public method call print, something like this (for a non template class):
std::ostream& MyClass::print(std::ostream& os) const
{
os << "Private One" << privateOne_ << endl;
os << "Private Two" << privateTwo_ << endl;
os.flush();
return os;
}
and then, outside the class (but in the same namespace)
std::ostream& operator<<(std::ostream& os, const MyClass& myClass)
{
return myClass.print(os);
}
I think it should work also for template class, but I haven't tested yet.
Here you go:
#include <cstdlib>
#include <iostream>
using namespace std;
template <class T>
T my_max(T a, T b)
{
if(a > b)
return a;
else
return b;
}
template <class classT>
class D
{
public:
D(classT in)
: d(in) {};
bool operator>(const D& rhs) const { return d > rhs.d;};
classT operator=(const D<classT>& rhs);
template<class classT> friend ostream& operator<< (ostream & os, const D<classT>& rhs);
private:
classT d;
};
template<class classT> ostream& operator<<(ostream& os, class D<typename classT> const& rhs)
{
os << rhs.d;
return os;
}
int main()
{
int i1 = 1;
int i2 = 2;
D<int> d1(i1);
D<int> d2(i2);
cout << my_max(d1,d2) << endl;
return 0;
}
I declared an overloaded operator as a friend in an Xcode C++ program
template <typename T> friend class list_template;
template <typename T> friend ostream& operator<< (ostream &, list_template<T> &);
It gives me an error on the second declaration that list_template has not been declared?
If I #include the file where list_template is declared I get more problems...
You're missing a global forward declaration of list_template, if I understand what you're trying to do:
MyClass.h
// forward declarator. must match definition in list_template.h
template<typename T> class list_template;
class MyClass
{
public:
MyClass() {};
virtual ~MyClass() {};
template<typename T> friend class list_template;
template<typename T> friend ostream& operator <<(ostream&, const list_template<T>&);
};
list_template.h
template<typename T>
class list_template
{
public:
list_template() {};
virtual ~list_template() {};
// declare friend ostream operator <<
friend ostream& operator << <>(ostream& os, const list_template<T>& lt);
};
// ostream insertion operator <<
template<typename T>
ostream& operator <<(ostream& os, const list_template<T>& lt)
{
// TODO: use lt here.
return os;
}
At least I think this is near where you were going.
I have read couple of the questions regarding my problem on StackOverflow.com now, and none of it seems to solve my problem. Or I maybe have done it wrong...
The overloaded << works if I make it into an inline function. But how do I make it work in my case?
warning: friend declaration std::ostream& operator<<(std::ostream&, const D<classT>&)' declares a non-template function
warning: (if this is not what you intended, make sure the function template has already been declared and add <> after the function name here) -Wno-non-template-friend disables this warning
/tmp/cc6VTWdv.o:uppgift4.cc:(.text+0x180): undefined reference to operator<<(std::basic_ostream<char, std::char_traits<char> >&, D<int> const&)'
collect2: ld returned 1 exit status
The code:
template <class T>
T my_max(T a, T b)
{
if(a > b)
return a;
else
return b;
}
template <class classT>
class D
{
public:
D(classT in)
: d(in) {};
bool operator>(const D& rhs) const;
classT operator=(const D<classT>& rhs);
friend ostream& operator<< (ostream & os, const D<classT>& rhs);
private:
classT d;
};
int main()
{
int i1 = 1;
int i2 = 2;
D<int> d1(i1);
D<int> d2(i2);
cout << my_max(d1,d2) << endl;
return 0;
}
template <class classT>
ostream& operator<<(ostream &os, const D<classT>& rhs)
{
os << rhs.d;
return os;
}
This is one of those frequently asked questions that have different approaches that are similar but not really the same. The three approaches differ in who you are declaring to be a friend of your function --and then on how you implement it.
The extrovert
Declare all instantiations of the template as friends. This is what you have accepted as answer, and also what most of the other answers propose. In this approach you are needlessly opening your particular instantiation D<T> by declaring friends all operator<< instantiations. That is, std::ostream& operator<<( std::ostream &, const D<int>& ) has access to all internals of D<double>.
template <typename T>
class Test {
template <typename U> // all instantiations of this template are my friends
friend std::ostream& operator<<( std::ostream&, const Test<U>& );
};
template <typename T>
std::ostream& operator<<( std::ostream& o, const Test<T>& ) {
// Can access all Test<int>, Test<double>... regardless of what T is
}
The introverts
Only declare a particular instantiation of the insertion operator as a friend. D<int> may like the insertion operator when applied to itself, but it does not want anything to do with std::ostream& operator<<( std::ostream&, const D<double>& ).
This can be done in two ways, the simple way being as #Emery Berger proposed, which is inlining the operator --which is also a good idea for other reasons:
template <typename T>
class Test {
friend std::ostream& operator<<( std::ostream& o, const Test& t ) {
// can access the enclosing Test. If T is int, it cannot access Test<double>
}
};
In this first version, you are not creating a templated operator<<, but rather a non-templated function for each instantiation of the Test template. Again, the difference is subtle but this is basically equivalent to manually adding: std::ostream& operator<<( std::ostream&, const Test<int>& ) when you instantiate Test<int>, and another similar overload when you instantiate Test with double, or with any other type.
The third version is more cumbersome. Without inlining the code, and with the use of a template, you can declare a single instantiation of the template a friend of your class, without opening yourself to all other instantiations:
// Forward declare both templates:
template <typename T> class Test;
template <typename T> std::ostream& operator<<( std::ostream&, const Test<T>& );
// Declare the actual templates:
template <typename T>
class Test {
friend std::ostream& operator<< <T>( std::ostream&, const Test<T>& );
};
// Implement the operator
template <typename T>
std::ostream& operator<<( std::ostream& o, const Test<T>& t ) {
// Can only access Test<T> for the same T as is instantiating, that is:
// if T is int, this template cannot access Test<double>, Test<char> ...
}
Taking advantage of the extrovert
The subtle difference between this third option and the first is in how much you are opening to other classes. An example of abuse in the extrovert version would be someone that wants to get access into your internals and does this:
namespace hacker {
struct unique {}; // Create a new unique type to avoid breaking ODR
template <>
std::ostream& operator<< <unique>( std::ostream&, const Test<unique>& )
{
// if Test<T> is an extrovert, I can access and modify *any* Test<T>!!!
// if Test<T> is an introvert, then I can only mess up with Test<unique>
// which is just not so much fun...
}
}
You can't declare a friend like that, you need to specify a different template type for it.
template <typename SclassT>
friend ostream& operator<< (ostream & os, const D<SclassT>& rhs);
note SclassT so that it doesn't shadow classT. When defining
template <typename SclassT>
ostream& operator<< (ostream & os, const D<SclassT>& rhs)
{
// body..
}
This worked for me without any compiler warnings.
#include <iostream>
using namespace std;
template <class T>
T my_max(T a, T b)
{
if(a > b)
return a;
else
return b;
}
template <class classT>
class D
{
public:
D(classT in)
: d(in) {};
bool operator>(const D& rhs) const {
return (d > rhs.d);
}
classT operator=(const D<classT>& rhs);
friend ostream& operator<< (ostream & os, const D& rhs) {
os << rhs.d;
return os;
}
private:
classT d;
};
int main()
{
int i1 = 1;
int i2 = 2;
D<int> d1(i1);
D<int> d2(i2);
cout << my_max(d1,d2) << endl;
return 0;
}
I think you shouldn't make friend in the first place.
You can create a public method call print, something like this (for a non template class):
std::ostream& MyClass::print(std::ostream& os) const
{
os << "Private One" << privateOne_ << endl;
os << "Private Two" << privateTwo_ << endl;
os.flush();
return os;
}
and then, outside the class (but in the same namespace)
std::ostream& operator<<(std::ostream& os, const MyClass& myClass)
{
return myClass.print(os);
}
I think it should work also for template class, but I haven't tested yet.
Here you go:
#include <cstdlib>
#include <iostream>
using namespace std;
template <class T>
T my_max(T a, T b)
{
if(a > b)
return a;
else
return b;
}
template <class classT>
class D
{
public:
D(classT in)
: d(in) {};
bool operator>(const D& rhs) const { return d > rhs.d;};
classT operator=(const D<classT>& rhs);
template<class classT> friend ostream& operator<< (ostream & os, const D<classT>& rhs);
private:
classT d;
};
template<class classT> ostream& operator<<(ostream& os, class D<typename classT> const& rhs)
{
os << rhs.d;
return os;
}
int main()
{
int i1 = 1;
int i2 = 2;
D<int> d1(i1);
D<int> d2(i2);
cout << my_max(d1,d2) << endl;
return 0;
}