How to use C++ explicit instantiation to hide implementation of template class or member

README.md

Rationale

This gist highlights the fact that implementation of C++ template symbols do not necessary have to be available where there are used.

File Layout

• foo.hh: Provide template class Foo declaration
• foo.cc: Provide template class Foo definition + explicit instantiation of Foo<int>
• bar.cc: Provide main function using Foo<Foo_TYPE> where FOO_TYPE is a macro with int as default value.

Usage

Run make to successfully build the main executable despite compiler does not know implementation of Foo::incr when compiling bar.cc

$ make
g++ -Wall -ansi -pedantic -Werror   -c -o foo.o foo.cc
g++ -Wall -ansi -pedantic -Werror   -c -o bar.o bar.cc
g++ foo.o bar.o -o main

Run CPPFLAGS="-DFOO_TYPE=double" make distclean all to see that bar.cc compilation passes but linkage doesn't.

$ CPPFLAGS="-DFOO_TYPE=double" make distclean all
rm -f foo.o bar.o
rm -f main
g++ -Wall -ansi -pedantic -Werror -DFOO_TYPE=double  -c -o foo.o foo.cc
g++ -Wall -ansi -pedantic -Werror -DFOO_TYPE=double  -c -o bar.o bar.cc
g++ foo.o bar.o -o main
bar.o: In function `main':
bar.cc:(.text+0x33): undefined reference to `Foo<double>::incr(double const&)'
collect2: error: ld returned 1 exit status
Makefile:7: recipe for target 'main' failed
make: *** [main] Error 1

bar.cc

#include "foo.hh"

#ifndef FOO_TYPE
# define FOO_TYPE int
#endif

int main() {
  Foo<FOO_TYPE> f;
  return f.incr(-1);
}

foo.cc

#include "foo.hh"

// definition of Foo::incr
template <typename T>
T Foo<T>::incr(const T& x) {
  return x + 1;
}

// explicit instantiation for type int
template class Foo<int>;

foo.hh

template <typename T>
class Foo {
public:
  T incr(const T& x);
};

Makefile

SOURCES = foo.cc bar.cc
CXXFLAGS = -Wall -ansi -pedantic -Werror

all: main

main: $(SOURCES:.cc=.o)
	$(CXX) $? -o $@

clean:
	$(RM) $(SOURCES:.cc=.o)

distclean: clean
	$(RM) main

depend:
	makedepend $(SOURCES)

# DO NOT DELETE

foo.o: foo.hh
bar.o: foo.hh