Combining several static libraries into one using CMake

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Given the most simple working example I can think of: 2 classes, a and b, where a depends on b . .

a.h

#ifndef A_H
#define A_H

class aclass
{
public:
    int method(int x, int y);
};

#endif

a.cpp

#include "a.h"
#include "b.h"

int aclass::method(int x, int y) {
    bclass b;
    return x * b.method(x,y);
}

b.h

#ifndef B_H
#define B_H

class bclass
{
public:
    int method(int x, int y);
};

#endif

b.cpp

#include "b.h"

int bclass::method(int x, int y) {
    return x+y;
}

main.cpp

#include "a.h"
#include <iostream>

int main()
{
    aclass a;
    std::cout << a.method(3,4) << std::endl;

    return 0;
}

It is possible to compile these into separate static libs, and then combine the static libs using a custom target.

cmake_minimum_required(VERSION 2.8.7)

add_library(b b.cpp b.h)
add_library(a a.cpp a.h)
add_executable(main main.cpp)

set(C_LIB ${CMAKE_BINARY_DIR}/libcombi.a)

add_custom_target(combined
        COMMAND ar -x $<TARGET_FILE:a>
        COMMAND ar -x $<TARGET_FILE:b>
        COMMAND ar -qcs ${C_LIB} *.o
        WORKING_DIRECTORY ${CMAKE_BINARY_DIR}
        DEPENDS a b
        )

add_library(c STATIC IMPORTED GLOBAL)
add_dependencies(c combined)

set_target_properties(c
        PROPERTIES
        IMPORTED_LOCATION ${C_LIB}
        )

target_link_libraries(main c)

It also works just fine using Apple’s libtool version of the custom target . . .

add_custom_target(combined
        COMMAND libtool -static -o ${C_LIB} $<TARGET_FILE:a> $<TARGET_FILE:b>
        WORKING_DIRECTORY ${CMAKE_BINARY_DIR}
        DEPENDS a b
        )

Still seams as though there should be a neater way . .

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