在 Xcode 上的 Swift 项目上使用 C++ 时出错：体系结构 x86_64 的未定义符号

Question

There are a lot of questions about this on the web but I couldn't solve my problem. I've been studying this for a few days.

I want run a simple C++ class on Swift project, to this I followed this tutorial: http://www.swiftprogrammer.info/swift_call_cpp.html . Basically I've followed the steps:

1. Create junk.cpp and junk.h files
2. Compile using g++ or/and clang++
3. Create.a file with: $ ar r libjunkcpp.a junk.o
4. ranlib libjunkcpp.a
5. Linked to Xcode in Build Phases -> Link Binary With Libraries -> Add

When I compiles, the follow errors occurs:

Undefined symbols for architecture x86_64:
  "A::getInt()", referenced from:
      _getIntFromCPP in wrapper.o
  "A::A(int)", referenced from:
      _getIntFromCPP in wrapper.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

junk.h

class A {
    public:
    A(int);
    int getInt();

    private:
    int m_Int;
};

junk.cpp

#include "junk.h"

A::A(int _i) : m_Int(_i) {}

int A::getInt() {
    return m_Int
}

wrapper.cpp

#include "junk.h"

extern "C" int getIntFromCPP() {
    // Create an instance of A, defined in
    // the library, and call getInt() on it:
    return A(1234).getInt();
}

bridge.h

int getIntFromCPP();

Answer 1

This is an issue that sometimes occurs when using C or C++ libraries that get bridged through Objective-C into Swift. The issue isn't with how the Archive (static library) is built -- rather, it's a problem with an overly-aggressive linker creating the application.

The problem is that the linker sees the binding symbols and the C++ symbols, but never sees it being called from anywhere because it is actually invoked from a different language (Swift). This ultimately results in the linker aggressively stripping all symbols that it doesn't believe are being used in an effort to save space, which effectively removes all of the C++ code.

When this happens, nm will report the correct symbols as existing in the archive, but the actual application will fail to execute due to the symbols not being found.

---

As for solutions, there are actually several questions/answers already on Stack Overflow that address this issue in varying degrees:

• How to disable C++ dead code stripping in xcode , where the solution is to use the linker flag -force_load to force that the static symbols are actually loaded.

• Keeping Xcode from stripping out unused symbols from a static library which suggests using __attribute__((constructor)) to trick the linker into thinking the symbols are needed on construction, thus keeping them in.

• Xcode Archive debug strip errors , which more broadly suggests toggling some Xcode configurations that may result in the symbols not being stripped. The suggestions in the answers here may conflict with the above two links, however.

It is difficult to provide a definitive and clear answer without more information on the existing setup, such as where wrapper.cpp is built, and how the bindings get used from within the Swift project. Given that this question is from 2018, I don't suspect there will be much more information on what this setup was at the time.

From the described symptoms, this sounds exactly like the issue my colleagues faced when doing C++/Swift bindings -- and the solution was ultimately to use -force_load which ensures that the archive is preserved in totality into the final application.