Basic doubt with Class template

[Orjan Westin]

If you are still looking for an answer, I posted an article explaining this just today. http://www.codeproject.com/Articles/236927/All-your-base64-are-different-to-us In short, there is a way, provided you know all template types you will be using. I'll copy the example I had in the article here:

-------------------------------------
-- In my_template.h file
...
template
T my_temp_func(const T& t);

---

-- In my_template.cpp file

#include "my_template.h"

// Definition
template
T my_temp_func(const T& t)
{
return t;
}

// Instantiation declaration
int my_temp_func(const int& t);

---

-- In using_my_template.cpp
#include "my_template.h"
...
int i = my_temp_func(4); // works
string s = my_temp_func("Abob"); // link error

In the example above, the declaration in the last line of my_template.cpp tells the compiler there’ll be a variant of the template function that uses int as template parameter. Okay, says the compiler, I’ll put an inline copy there. Since the generic definition is right there in the same compilation unit (ie my_template.cpp), this is something the compiler can do – it has all the information it needs. The result of that is that in the compiled file (probably called my_template.obj) there is now a function that has the signature int my_temp_func(const int& t). This is a fully defined specialisation of a template function, so to the linker it looks just like a normal function. However, the linker won’t be able to find a string specialisation, so this will generate a linker error. In

[Bram van Kampen]

Your basic problem appears to be to understand how Header Files, cpp Files, the Compiler and the Linker work. First the Compiler. The Compiler builds an Object File. This is a Binary representation of a cpp File. It containd the CPP Code you wrote, converted to machine code as well as a whole raft of information needed later. The Compiler contains a thing, called a Pre-Processor. It prepares your CPP file before actual compilation. Conceptually it produces a New Source File. It copies your file Line by Line. but if it finds a statement like #include "MyBeautifulHdr.h", it does not copy the include statement, but, instead writes the entire contents of "MyBeautifulHdr.h" at the location of the '#include' statement. When it encounters a #define FOO 1 statement, it undertakes to replace henceforth every future occurrence of 'FOO' with 1. It goes on like that at infinitum, and the final file offered to the compiler has all '#includes, #defines, #if's etc resolved. At that stage the compiler does not know ( and does not care) from what file what item came from, (except for error reporting). It just builds the Object File, (if it Can) if it cannot, you get an error. Each of the Object Files generated roughly correspond to the code you wrote in a cpp file. An extern declaration is a promise to the compiler that somewhere in an other file or library, an item of that specification exists. The Linker tries to build an executable out of all those object files. In doing so, it may also include Object files written by others, e.g. those written by Microsoft, which came with your compiler package. To make that process more efficient, multiple obj files can be gathered together into a library (.lib) file If the linker cannot find a named item, or, finds more than one, the linker will fail building the executable. Getting a Full understanding of how your tools work will answer your queery. regards, :)

Bram van Kampen