CODEWITHSUNDEEP
c++templatescompilation
Walter
Walter

Reputation: 45414

selective compilation of template

Is there a way to pre-compile some template instantinations (of a class template) but not all, so that the remaining are compiled when instantinated (w/o errors at linking)?

To demonstrate the issue, consider this example:

// file.h
template<typename T> class object { /* lots of code */ };

// file.inc
template<typename T>
object::object(const T*data) { /* ... */ }
    // and more: definitions of all non-inline functionality of class object<>

// file.cc to be compiled and linked
#include "file.h"
#include "file.inc" 

template struct<int>;
template struct<double>;

// user.cc
#include "user.h"
#include "file.h"

object<double> x{0.4}                              // okay: uses pre-compiled code
object<user_defined> z(user_defined{"file.dat"});  // error: fails at linking

If instead the user #includes "file.inc"

// user.cc
#include "user.h"
#include "file.h"
#include "file.inc"

object<double>  x{0.4}                            // error: duplicate code 
object<user_defined> z(user_defined{"file.dat"}); // okay

the compiler will find another compiled version due to the pre-compiled code from file.cc.

So how can I avoid both of these problems? I reckon a related question would be "how can I specify that a pre-compiled header fully compiles a template for certain template parameters (only)?"

Upvotes: 0

Views: 66

Answers (1)

Vittorio Romeo
Vittorio Romeo

Reputation: 93264

You can use extern template to prevent a particular instantiation of a template in a given TU. 

// src0.cpp

template class foo<int>;
    // Oblige instantiation of `foo<int>` in this TU


// src1.cpp

extern template class foo<int>;
    // Prevent instantiation of `foo<int>` in this TU

As long as src0 and src1 are linked together, your program will work.

Upvotes: 1

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