std :: bad_function_call自调用递归lambda但没有提示

Question

I'm using lldb and llvm (clang 6.0) on OSX. The following code throws an std::bad_function_call at line 30:

std::function<void ( const std::shared_ptr<Node>, unsigned int )> find_next;
find_next = [=]( const std::shared_ptr<Node> node_to, unsigned int len )
{
    for ( const auto rhs : _edges )
    {  
        assert(node_to);
        assert(rhs.from);
        if ( (*node_to) == (*rhs.from) ) 
        {
            len++;
            find_next ( rhs.from, len );   // line 30
        }
    }
};

This is the caller, right below the definition of the lambda:

for ( const auto lhs : _edges )
{
    unsigned int len = 0;
    const auto from = lhs.to;
    find_next ( from, len );
}

And this is the lldb output:

libc++abi.dylib: terminating with uncaught exception of type std::__1::bad_function_call: std::exception
Avg Path Length: Process 4369 stopped
* thread #1: tid = 0x17220, 0x00007fff8c2bd282 libsystem_kernel.dylib`__pthread_kill + 10, queue = 'com.apple.main-thread', stop reason = signal SIGABRT
    frame #0: 0x00007fff8c2bd282 libsystem_kernel.dylib`__pthread_kill + 10
libsystem_kernel.dylib`__pthread_kill + 10:
-> 0x7fff8c2bd282:  jae    0x7fff8c2bd28c            ; __pthread_kill + 20
   0x7fff8c2bd284:  movq   %rax, %rdi
   0x7fff8c2bd287:  jmp    0x7fff8c2b8ca3            ; cerror_nocancel
   0x7fff8c2bd28c:  retq  

Running a backtrace:

(lldb) bt
* thread #1: tid = 0x17220, 0x00007fff8c2bd282 libsystem_kernel.dylib`__pthread_kill + 10, queue = 'com.apple.main-thread', stop reason = signal SIGABRT
  * frame #0: 0x00007fff8c2bd282 libsystem_kernel.dylib`__pthread_kill + 10
    frame #1: 0x00007fff8bca24c3 libsystem_pthread.dylib`pthread_kill + 90
    frame #2: 0x00007fff918ebb73 libsystem_c.dylib`abort + 129
    frame #3: 0x00007fff8ba8ea21 libc++abi.dylib`abort_message + 257
    frame #4: 0x00007fff8bab69b9 libc++abi.dylib`default_terminate_handler() + 243
    frame #5: 0x00007fff8c3f96db libobjc.A.dylib`_objc_terminate() + 124
    frame #6: 0x00007fff8bab40a1 libc++abi.dylib`std::__terminate(void (*)()) + 8
    frame #7: 0x00007fff8bab3b30 libc++abi.dylib`__cxa_throw + 121
    frame #8: 0x000000010009f453 extras`std::__1::function<void (this=0x0000000100400370, __arg=shared_ptr<cgpp::Node> at 0x00007fff5fbfec28, __arg=1)>::operator()(std::__1::shared_ptr<cgpp::Node>, unsigned int) const + 131 at functional:1753
    frame #9: 0x000000010009edfd extras`cgpp::ConceptualGraph::avgPathLength(this=0x0000000100400360, node_to=<unavailable>, len=1) const::$_0::operator()(std::__1::shared_ptr<cgpp::Node>, unsigned int) const + 925 at ConceptualGraphEXTRAS.cpp:30
    frame #10: 0x000000010009e992 extras`std::__1::__function::__func<cgpp::ConceptualGraph::avgPathLength() const::$_0, std::__1::allocator<cgpp::ConceptualGraph::avgPathLength() const::$_0>, void (std::__1::shared_ptr<cgpp::Node>, unsigned int)>::operator()(std::__1::shared_ptr<cgpp::Node>&&, unsigned int&&) [inlined] decltype(this=0x0000000100400360, __f=0x0000000100400360, __args=0x00007fff5fbfef50, __args=0x00007fff5fbfeeb4) const::$_0&>(fp)(std::__1::forward<std::__1::shared_ptr<cgpp::Node>, unsigned int>(fp0))) std::__1::__invoke<cgpp::ConceptualGraph::avgPathLength() const::$_0&, std::__1::shared_ptr<cgpp::Node>, unsigned int>(cgpp::ConceptualGraph::avgPathLength() const::$_0&&&, std::__1::shared_ptr<cgpp::Node>&&, unsigned int&&) + 119 at __functional_base:413
    frame #11: 0x000000010009e91b extras`std::__1::__function::__func<cgpp::ConceptualGraph::avgPathLength(this=0x0000000100400350, __arg=0x00007fff5fbfef50, __arg=0x00007fff5fbfeeb4) const::$_0, std::__1::allocator<cgpp::ConceptualGraph::avgPathLength() const::$_0>, void (std::__1::shared_ptr<cgpp::Node>, unsigned int)>::operator()(std::__1::shared_ptr<cgpp::Node>&&, unsigned int&&) + 91 at functional:1370
    frame #12: 0x000000010009f48d extras`std::__1::function<void (this=0x00007fff5fbff340, __arg=shared_ptr<cgpp::Node> at 0x00007fff5fbfef50, __arg=0)>::operator()(std::__1::shared_ptr<cgpp::Node>, unsigned int) const + 189 at functional:1755
    frame #13: 0x000000010009cc89 extras`cgpp::ConceptualGraph::avgPathLength(this=0x0000000100304e38) const + 1065 at ConceptualGraphEXTRAS.cpp:40
    frame #14: 0x00000001000029e6 extras`main + 3334 at extras.cpp:37
    frame #15: 0x00007fff869b15c9 libdyld.dylib`start + 1
    frame #16: 0x00007fff869b15c9 libdyld.dylib`start + 1

I capture everything by value, as the iterator seems to invalidate the pointer if I capture by reference.

What I do not understand, is that the recursive call at line 30, I get the hint from lldb ( frame #9: 0x000000010009edfd ) that node_to is not available. However the assertion doesn't catch this, but only instead an std::bad_function_call is thrown, which if I understand correctly, means that the function I am calling is invalid?

EDIT: I made a minified example: on ideone.com

Answer 1

You're capturing find_next by value, before assigning the lambda to it, while it's still empty.

Capturing by reference should work, as long as you don't need to copy the function and use it after destroying the original.