creating a static library using g++ -std=c++11 using templates

Question

While creating a static library using c++11, it fails during linking I think.

I can create a static library and link to it using the information in How to create a static library with g++? with ordinary c++, but if I try to follow the steps using c++11 features, it fails during the linking.

test.cpp:

#include <iostream> #include <vector> #include "libtestlib.h" using namespace std; int main() { itest n1={{1,2,3,4}, {5,6,7,8}, {9,0,1,2}}; cout << "testing...\n"; test_print(n1); return 0; }

libtestlib.h:

#ifndef testlib #define testlib #include <vector> using namespace std; typedef vector<vector<double>> dtest; typedef vector<vector<int>> itest; template <typename testtype> void test_print(testtype); #endif

libtestlib.cpp:

#include <iostream> #include "libtestlib.h" using namespace std; template <typename testtype> void test_print(testtype &t) { int m=t.size(); int n=t[0].size(); for(int i=0; i<m; i++) { for(int j=0; j<n; j++) cout << t[i][j] << " "; cout << endl; } cout << endl; }

this is the output I'm getting:

$ g++ -std=c++11 -c libtestlib.cpp $ ar rvs libtestlib.a libtestlib.o r - libtestlib.o $ g++ -std=c++11 test.cpp libtestlib.a /tmp/cccJ7SXZ.o:test.cpp:(.text+0x1af): undefined reference to `void test_print<std::vector<std::vector<int, std::allocator<int> >, std::allocator<std::vector<int, std::allocator<int> > > > >(std::vector<std::vector<int, std::allocator<int> >, std::allocator<std::vector<int, std::allocator<int> > > >)' /tmp/cccJ7SXZ.o:test.cpp:(.text+0x1af): relocation truncated to fit: R_X86_64_PC32 against undefined symbol `void test_print<std::vector<std::vector<int, std::allocator<int> >, std::allocator<std::vector<int, std::allocator<int> > > > >(std::vector<std::vector<int, std::allocator<int> >, std::allocator<std::vector<int, std::allocator<int> > > >)' collect2: error: ld returned 1 exit status

Your library is essentially empty since that is a templated function it won't have any instantiations. It should be in the libtestlib.h header or included into it. Once you do that you can statically compile it. — estabroo
– estabroo, Commented Apr 12, 2019 at 21:56
Thanks for your response @estabroo. Can you give an example of what you mean? — Third
– Third, Commented Apr 12, 2019 at 22:36
isocpp.org/wiki/faq/templates#templates-defn-vs-decl gives a good explanation. And for an example take your code above, move what's in libtestlib.cpp into libtestlib.h and compile it. g++ --std=c++11 -static -o test_it test.cpp — estabroo
– estabroo, Commented Apr 12, 2019 at 22:48

user4442671 · Accepted Answer · 2019-04-12 22:03:00Z

Since it seems you are supporting operations on a limited number of types, the classic "Just do it in a header" answer to this problem is not necessarily the best one.

You can get the best of both worlds by explicitely exporting a symbol for each implementation, but delegate the implementation to a template within the library:

libtestlib.h:

#ifndef testlib #define testlib #include <vector> using namespace std; typedef vector<vector<double>> dtest; typedef vector<vector<int>> itest; void test_print(dtest&); void test_print(itest&);

libtestlib.cpp:

#include <iostream> #include "libtestlib.h" using namespace std; namespace { template <typename testtype> void test_print_impl(testtype &t) { int m=t.size(); int n=t[0].size(); for(int i=0; i<m; i++) { for(int j=0; j<n; j++) cout << t[i][j] << " "; cout << endl; } cout << endl; } } void test_print(dtest& val) { test_print_impl(val); } void test_print(itest& val) { test_print_impl(val); }

Mind you, for a small function like this, it's probably not worth the effort, and just inlining the template code in the header is just fine. At what point does the complexity of a function and the scope of its dependencies warrant this is a bit of a judgement call.

Thanks @Frank! Your solution works for me, but of course my real situation is not as simple/small as this example. :)
@Frank nice and actually gives you the option to build a static library, whereas in the header doesn't really lend itself to doing that.

estabroo · Accepted Answer · 2019-04-12 22:50:43Z

Putting example requested from comment on the question here as code looked really ugly in a comment. Combined libtestlib.h and libtestlib.cpp

#ifndef testlib #define testlib #include <vector> #include <iostream> using namespace std; typedef vector<vector<double>> dtest; typedef vector<vector<int>> itest; template <typename testtype> void test_print(testtype &t) { int m=t.size(); int n=t[0].size(); for(int i=0; i<m; i++) { for(int j=0; j<n; j++) cout << t[i][j] << " "; cout << endl; } cout << endl; } #endif

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