Question

Technique 1 -> Results in error

I have three files MyType.h, MyType.cpp & main.cpp

MyType.h

#ifndef MYTYPE_H
#define MYTYPE_H

#include<iostream>
using namespace std;

template <class T,int iMax>

class A{
    T iData;
public:
    void vSetData(T iPar1);
    void vDisplayData();
};
#endif

MyType.cpp

#include"MyType.h"

void A::vSetData(T iPar1){
    if(iPar1 <= iMax)
        iData = iPar1;
}
void A::vDisplayData(){
    cout<<"\nData is: "<<iData<<endl;
}

main.cpp

#include"MyType.h"

typedef A<int,20> MyType;

int main(){
    int x = 12;
    MyType obj;
    obj.vSetData(12);
    obj.vDisplayData();
    return 0;
}

ERRORS: 10 errors. They are as follows:-

• mytype.cpp(2) : error C2955: ‘A’ : use of class template requires template argument list
• mytype.h(9) : see declaration of ‘A’
• mytype.cpp(2) : error C2955: ‘A’ : use of class template requires template argument list
• mytype.h(9) : see declaration of ‘A’
• mytype.cpp(2) : error C2065: ‘T’ : undeclared identifier
• mytype.cpp(2) : error C2146: syntax error : missing ‘)’ before identifier ‘iPar1’
• mytype.cpp(2) : error C2761: ‘void A::vSetData(T)’ : member function redeclaration not allowed
• mytype.cpp(2) : error C2059: syntax error : ‘)’
• mytype.cpp(2) : error C2143: syntax error : missing ‘;’ before ‘{‘
• mytype.cpp(2) : error C2447: ‘{‘ : missing function header (old-style formal list?)
• mytype.cpp(6) : error C2955: ‘A’ : use of class template requires template argument list
• mytype.h(9) : see declaration of ‘A’
• mytype.cpp(6) : error C2509: ‘vDisplayData’ : member function not declared in ‘A’
• mytype.h(9) : see declaration of ‘A

---

Technique 2 -> Works fine.

AboveCodeInOneFile.cpp

#include<iostream>
using namespace std;

template <class T,int iMax>

class A{
    T iData;
public:
    void vSetData(T iPar1){
        if(iPar1 <= iMax)
            iData = iPar1;
    }
    void vDisplayData(){
        cout<<"\nData is: "<<iData<<endl;
    }
};

typedef A<int,20> MyType;

int main(){
    int x = 12;
    MyType obj;
    obj.vSetData(12);
    obj.vDisplayData();
    return 0;
}

Please let me know what mistake i am doing in Technique 1

Answer 1

Each method definition needs the full class name decoration above it:

template <typename T>
void A<T>::vSetData(T iPar1){
    if(iPar1 <= iMax)
        iData = iPar1;
}

template <typename T>
void A<T>::vDisplayData(){
    cout<<"\nData is: "<<iData<<endl;
}

When you do this however you’ll get other compile errors, for the reasons Abhineet outlined. To fix this, you could put the definitions I wrote into the header file, beneath the class declaration, or you could put it in another header (MyType_Inc.h) and include /that/ header in the class declaration header