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