What is the syntax for object instantiation in C++? - c++

Question

when I compile the following c++ code:
#include "ConstantList.h"
using namespace std;
int main() {
ConstantList* cl = new ConstantList();
//do something with cl
delete cl;
cl = NULL;
return 0;
}
The compiler gives me the error:
Undefined symbols:
"ConstantList::~ConstantList()", referenced from:
_main in ccNfeeDU.o
"ConstantList::ConstantList()", referenced from:
_main in ccNfeeDU.o
ld: symbol(s) not found
collect2: ld returned 1 exit status
Am I not getting the syntax right for instantiating an object?
My ConstantList.h file looks like this:
#ifndef ConstantList_h
#define ConstantList_h
#include <string>
#include "Token.h"
using namespace std;
class ConstantListTail;
class ConstantList {
public:
ConstantList();
~ConstantList();
std::string toString();
void push_back(Token*);
void push_back(ConstantListTail*);
private:
Token* termString;
ConstantListTail* constantListTail;
};
#endif
Any help is greatly appreciated!

Answer 1

Your syntax is correct, because you are getting a linker error, not a compiler error. This error means that you are compiling your main without the source of the ConstantList.cpp, or linking without a reference to ConstantList.o
Compiling with this command should fix the error:
g++ collect2.cpp ConstantList.cpp
(I am assuming that the file with your main function is called collect2.cpp).

Answer 2

"undefined symbol" means you have declared the identifier (in this case the destructor), and it's used, but as far as the linker knows you have not defined it
add a definition somewhere, and make sure the compiled version is in one of the files the linker links
re "syntax for instantiation", unfortunately there is no dedicated syntax for that in C++
instead the functional cast notation is used for constructor invocations
perhaps the closest you get to a pure instantiation syntax is the new expression
re
using namespace std;
in a header file: don't.
for example, the standard library defines something called distance. what are the chances that some code that includes the header will have its own distance, and get a name collision? much higher than zero.
this doesn't mean you should never have using namespace std; in a header file, but you should never have it in the global namespace in a header file. and for other namespaces, be very aware of what that does, namely offering all the standard library names as part of also that namespace.