Inheriting from a template class in c++, fail to compile - c++

Question

My question is that, I have a template class template<class T> AList as base, and I wanna get a derived class from the template, i.e. get class BList: public AList<mydefinedtype> without much modification.
alist.h
#ifndef alist_h
#define alist_h
template<class T> class AList
{
public:
AList(){
arr = new T[20];
numitems = 0;
};
void append(T value);
private:
T *arr;
int numitems;
};
#endif /* alist_h */
alist.cpp
#include "alist.h"
template<class T> void AList<T>::append(T value)
{
arr[numitems] = value;
++numitems;
return;
}
blist.h
#include "alist.cpp"
#include <string>
using namespace std;
typedef struct
{
string a, b;
int key;
} record;
class BList: public AList<record>{
public:
void test(void){
cout << "this is from BList" << endl;
}
};
blist.cpp
#include "blist.h"
main.cpp
#include <iostream>
#include "blist.cpp"
using namespace std;
int main(){
record testRecord[3];
testRecord[0] = {"Mark", "A", 1};
testRecord[1] = {"Anla", "B", 2};
testRecord[2] = {"Cindy", "C", 3};
BList blist = BList();
for(auto i: testRecord){
// blist.append(i); // will compile error
blist.test();
}
return 0;
}
It will fail as follows, I wonder how to compile or how to fix the bug.
error info
Undefined symbols for architecture x86_64:
"AList<record>::append(s)", referenced from:
_main in main.o
ld: symbol(s) not found for architecture x86_64
Not sure where comes from the issue.

Answer 1

// Example program
#include <iostream>
#include <string>
struct record{
int a;
};
template<class T>
class AList{
public:
AList()=default;
void append(T value){}
};
template<class T>
class BList:public AList<T>{
public:
void test(void){}
};
int main()
{
BList<record> blist;
record recordarr[3] ;
// some initialization
for(auto i:recordarr){
blist.append(i);
blist.test();
}
}
The problem you have is that the AList() constructor, append(T) and test() are only declared but not defined. The above code should compile.

Answer 2

You should put your template classes entirely in header files. See this question and this C++ FAQ for details on why.
You should also never #include .cpp files. You should only ever #include header files.
Below I have your code after the required modifications to make it compile. I also removed your memory leak.
alist.h:
#ifndef alist_h
#define alist_h
template<class T> class AList {
public:
AList() {
arr = new T[20];
numitems = 0;
};
~AList() {
delete[] arr;
}
void append(T value) {
arr[numitems] = value;
++numitems;
}
private:
T *arr;
int numitems;
};
#endif /* alist_h */
blist.h:
#ifndef blist_h
#define blist_h
#include "alist.h"
#include <string>
using namespace std;
typedef struct {
string a, b;
int key;
} record;
class BList: public AList<record> {
public:
void test(void) {
cout << "this is from BList" << endl;
}
};
#endif /* blist_h */
main.cpp:
#include <iostream>
#include "blist.h"
using namespace std;
int main() {
record testRecord[3];
testRecord[0] = {"Mark", "A", 1};
testRecord[1] = {"Anla", "B", 2};
testRecord[2] = {"Cindy", "C", 3};
BList blist = BList();
for (auto i: testRecord) {
blist.append(i);
blist.test();
}
return 0;
}
Summary of changes
I made the following changes:
Moved body of AList::append into alist.h, and deleted alist.cpp
Added AList destructor to free the dynamically allocated memory allocated in AList::AList
In blist.h, included alist.h instead of alist.cpp
Deleted blist.cpp
In main.cpp, included blist.h instead of blist.cpp