C++: Compilation Errors for Custom Coded Generic Container?

Question

The following is based on the code that I posted on this thread. Aside from the obvious bugs, I get the following compilation errors? Any idea why?

The odd thing is that this only occurs for the template class. If I add another non template class to the same .h and .cpp file of the template class and try to instantiate just the non template class, no error is generated.

Scenario A:

I get the compilation error " error C2659: '=' : overloaded function as left operand " for the following code:

StdVector<int> a();
StdVector<int> b();
a = b;

Scenario B:

I get the compilation error " error C2664: '__thiscall StdVector::StdVector(const class StdVector &)' : cannot convert parameter 1 from 'class StdVector (__cdecl *)(void)' to 'const class StdVector &' " for the following code:

StdVector<int> a();
StdVector<int> b(a);

Scenario C:

I get the compilation error "error LNK2001: unresolved external symbol "public: __thiscall StdVector::~StdVector(void)" (??1?$StdVector@H@@QAE@XZ) " for the following code:

StdVector<int> a;

score 4 · Answer 1 · answered Nov 28 '09 at 15:28

4

StdVector<int> a();
StdVector<int> b();

These are function declarations. Omit the parenthesis to declare a default-constructed instance.

answered Nov 28 '09 at 15:28

UncleBens

40,819
6
57
90

score 1 · Answer 2 · answered Nov 28 '09 at 15:29

1

You are declaring a and b to be functions returning StdVector. I assume that's not what you want. What does it mean to assign a function to a function? Maybe you meant:

StdVector<int> a;

answered Nov 28 '09 at 15:29

Richard Pennington

19,673
4
43
72

I have edited the post for the error when I tried what you suggested out. – Lopper Nov 28 '09 at 15:37

score 1 · Answer 3 · edited May 23 '17 at 12:26

As the previous answers have said, in scenarios A and B you are actually declaring functions and not instantiating object instances. This is known as "C++'s most vexing parse". See this question for more discussion.

It is hard to tell why scenario C doesn't work without seeing the actual source code for your class, but the linker is telling you that it is unable to find the destructor for StdVector. It says it is an "unresolved external symbol", which means that when it was compiling it didn't have the definition and it either assumed, or was explicitly told, that the definition is in some other file. When it came time to link all of the object files into a single executable, it couldn't find the object code for the destructor, and that is why you are getting that error. That doesn't give you an easy solution for the problem, but at least you have an idea where to start looking.

I strongly suspect that this is due to the VC++6 compiler running under a virtualized environment. Will confirm this! — Lopper, Nov 29 '09 at 00:37

score 1 · Accepted Answer · answered Nov 29 '09 at 02:41

1

If I am not mistaken, templates cannot be exported. That is why I am getting the linker error in Scenario C.

answered Nov 29 '09 at 02:41

Lopper

3,499
7
38
57

C++: Compilation Errors for Custom Coded Generic Container?

4 Answers4