templates and g++

For the following code template_eg.cc:

Code:

---

template<typename T>
class MyArray
{
public:
  MyArray();
  MyArray(MyArray& copy);
  MyArray& operator=(MyArray& copy);
};

class MyData
{
public:
  MyData(MyArray<int>& x, MyArray<int>& y);
  const MyArray<int>& x();
  const MyArray<int>& y();
};

MyArray<int> read_data(int*, char**);
void make_changes(MyData* edit);

int main(int argc, char* argv[])
{
  const MyArray<int> read_x = read_data(&argc, argv);
  const MyArray<int> read_y = read_data(&argc, argv);

  MyData user_data(read_x, read_y);
  MyData edit_buffer(user_data);
  make_changes(&edit_buffer);
}

---

Why when I complie I get the following errors:

Code:

---

coletek@spamisgood:~/sandbox/cc> g++ template_eg.cc -o template_eg
template_eg.cc: In function ‘int main(int, char**)’:
template_eg.cc:23: error: no matching function for call to ‘MyArray<int>::MyArray(MyArray<int>)’
template_eg.cc:6: note: candidates are: MyArray<T>::MyArray(MyArray<T>&) [with T = int]
template_eg.cc:24: error: no matching function for call to ‘MyArray<int>::MyArray(MyArray<int>)’
template_eg.cc:6: note: candidates are: MyArray<T>::MyArray(MyArray<T>&) [with T = int]
template_eg.cc:26: error: no matching function for call to ‘MyData::MyData(const MyArray<int>&, const MyArray<int>&)’
template_eg.cc:13: note: candidates are: MyData::MyData(MyArray<int>&, MyArray<int>&)
template_eg.cc:11: note:                MyData::MyData(const MyData&)

---

I new to using templates. Thx.

So the line

Code:

---

const MyArray<int> read_x = read_data(&argc, argv);

---

calls the copy constructor -- you have a temporary MyArray<int> over there on the right. So you should make a copy constructor that works with a temporary object. Hint: that would like

Code:

---

MyArray(const MyArray &array_to_be_copied);

---

Ok, some fix is:

Code:

---

MyData(MyArray<int>& x, MyArray<int>& y);

---

should be

Code:

---

MyData(const MyArray<int>& x, const MyArray<int>& y);

---

for const-correctness, however still can't fix the "no matching function for call to ..." errors. Any ideas people?

Ok, so after fixing the code to:

Code:

---

template<typename T>
class MyArray
{
public:
  MyArray();
  //MyArray(MyArray& copy);                                             
  MyArray(const MyArray& copy);
  MyArray& operator=(MyArray& copy);
};

class MyData
{
public:
  //MyData(MyArray<int>& x, MyArray<int>& y);                                 
  MyData(const MyArray<int>& x, const MyArray<int>& y);
  const MyArray<int>& x();
  const MyArray<int>& y();
};

MyArray<int> read_data(int*, char**);
void make_changes(MyData* edit);

int main(int argc, char* argv[])
{
  const MyArray<int> read_x = read_data(&argc, argv);
  const MyArray<int> read_y = read_data(&argc, argv);

  MyData user_data(read_x, read_y);
  MyData edit_buffer(user_data);
  make_changes(&edit_buffer);
}

---

I'm now getting:

Code:

---

coletek@spamisgood:~/sandbox/cc> g++ template_eg.cc -o template_eg
/tmp/ccwMgmQo.o: In function `main':
template_eg.cc:(.text+0x30): undefined reference to `read_data(int*, char**)'
template_eg.cc:(.text+0x48): undefined reference to `read_data(int*, char**)'
template_eg.cc:(.text+0x64): undefined reference to `MyData::MyData(MyArray<int> const&, MyArray<int> const&)'
template_eg.cc:(.text+0x70): undefined reference to `make_changes(MyData*)'
collect2: ld returned 1 exit status

---

Arrr... where I'm I going wrong?

Quote:

---

Originally Posted by coletek

Ok, some fix is:

Code:

---

MyData(MyArray<int>& x, MyArray<int>& y);

---

should be

Code:

---

MyData(const MyArray<int>& x, const MyArray<int>& y);

---

for const-correctness, however still can't fix the "no matching function for call to ..." errors. Any ideas people?

---

That bears no resemblance to my hint, so it doesn't surprise me that it doesn't do anything to help. I see that you got it down below, though.

Quote:

---

Originally Posted by coletek

Ok, so after fixing the code to:

Code:

---

template<typename T>
class MyArray
{
public:
  MyArray();
  //MyArray(MyArray& copy);                                             
  MyArray(const MyArray& copy);
  MyArray& operator=(MyArray& copy);
};

class MyData
{
public:
  //MyData(MyArray<int>& x, MyArray<int>& y);                                 
  MyData(const MyArray<int>& x, const MyArray<int>& y);
  const MyArray<int>& x();
  const MyArray<int>& y();
};

MyArray<int> read_data(int*, char**);
void make_changes(MyData* edit);

int main(int argc, char* argv[])
{
  const MyArray<int> read_x = read_data(&argc, argv);
  const MyArray<int> read_y = read_data(&argc, argv);

  MyData user_data(read_x, read_y);
  MyData edit_buffer(user_data);
  make_changes(&edit_buffer);
}

---

I'm now getting:

Code:

---

coletek@spamisgood:~/sandbox/cc> g++ template_eg.cc -o template_eg
/tmp/ccwMgmQo.o: In function `main':
template_eg.cc:(.text+0x30): undefined reference to `read_data(int*, char**)'
template_eg.cc:(.text+0x48): undefined reference to `read_data(int*, char**)'
template_eg.cc:(.text+0x64): undefined reference to `MyData::MyData(MyArray<int> const&, MyArray<int> const&)'
template_eg.cc:(.text+0x70): undefined reference to `make_changes(MyData*)'
collect2: ld returned 1 exit status

---

Arrr... where I'm I going wrong?

---

I agree with your compiler: you do not, after all, have a function called read_data or make_changes. Just declaring the function name is not enough, but you need to actually write the functions themselves.

Of course, now complies with:

Code:

---

template<typename T>
class MyArray
{
public:
  MyArray();
  //MyArray(MyArray& copy);                                             
  MyArray(const MyArray& copy);
  MyArray& operator=(MyArray& copy);
};

class MyData
{
public:
  //MyData(MyArray<int>& x, MyArray<int>& y);                                 
  MyData(const MyArray<int>& x, const MyArray<int>& y) {};
  const MyArray<int>& x();
  const MyArray<int>& y();
};

//MyArray<int> read_data(int*, char**);
//void make_changes(MyData* edit);
MyArray<int> read_data(int*, char**) {};
void make_changes(MyData* edit) {};

int main(int argc, char* argv[])
{
  const MyArray<int> read_x = read_data(&argc, argv);
  const MyArray<int> read_y = read_data(&argc, argv);

  MyData user_data(read_x, read_y);
  MyData edit_buffer(user_data);
  make_changes(&edit_buffer);
}

---

But only because G++ is so lenient regarding return values. After all, read_data claims to return a MyArray<int> but returns nothing! This is most definitely undefined.

Tip: if you just want to make certain that code is semantically correct, but don't want to provide all implementations, you can use the -fsyntax-only switch to G++. In this mode, the compiler checks that your code is valid and that all templates can be instantiated, but it doesn't produce any output or try to link the file.

Quote:

---

Originally Posted by CornedBee

But only because G++ is so lenient regarding return values.

---

I've notice g++ is lenient on a few things, any idea why that is?

Because it has to cope with endless amounts of legacy code.