Question

The following code works.

/* hello.cc */

#include <iostream>
#include <vector>

void vec_print()
{
    std::vector<int> is(10, 1);

    for (size_t i = 0; i < is.size(); ++i)
        std::cout << is[i] << " ";
    std::cout << std::endl;
}

/* main.cc */

void vec_print();

int main()
{
    vec_print();
}

When i complie this with

g++ -fno-implicit-templates -Wall -Wextra -c hello.cc
g++ -fno-implicit-templates -Wall -Wextra -c main.cc
g++ hello.o main.o -o hello

I get no warnings and it compiles and runs fine. I am under the impression this shouldn’t happen. I am using gcc 4.4.5.

Answer 1

That option doesn’t prevent all template instantiations, just non-inline ones:

-fno-implicit-templates
Never emit code for non-inline templates which are instantiated implicitly (i.e. by use); only emit code for explicit instantiations. See Template Instantiation, for more information.

The compiler inlines all the vector member functions used in your code, so compilation succeeds.

The following will fail when compiled with that option, since it requires a non-inline instantiation:

#include <vector>

int main()
{
    auto f = &std::vector<int>::resize;
}