Remove never-run call to templated function, get allocation error on run-time

Question

I have a piece of templated code that is never run, but is compiled. When I remove it, another part of my program breaks.

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First off, I'm a bit at a loss as to how to ask this question. So I'm going to try throwing lots of information at the problem.

Ok, so, I went to completely redesign my test project for my experimental core library thingy. I use a lot of template shenanigans in the library. When I removed the "user" code, the tests gave me a memory allocation error. After quite a bit of experimenting, I narrowed it down to this bit of code (out of a couple hundred lines):

void VOODOO(components::switchBoard &board) {   
     board.addComponent<using_allegro::keyInputs<'w'> >();
}

Fundementally, what's weirding me out is that it appears that the act of compiling this function (and the template function it then uses, and the template functions those then use...), makes this bug not appear. This code is not being run. Similar code (the same, but for different key vals) occurs elsewhere, but is within Boost TDD code.

I realize I certainly haven't given enough information for you to solve it for me; I tried, but it more-or-less spirals into most of the code base. I think I'm most looking for "here's what the problem could be", "here's where to look", etc. There's something that's happening during compile because of this line, but I don't know enough about that step to begin looking.

Sooo, how can a (presumably) compilied, but never actually run, bit of templated code, when removed, cause another part of code to fail?

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Error:

Unhandled exceptionat 0x6fe731ea (msvcr90d.dll) in Switchboard.exe:
0xC0000005: Access violation reading location 0xcdcdcdc1.

Callstack:

operator delete(void * pUser Data)
allocator< class name related to key inputs callbacks >::deallocate
vector< same class >::_Insert_n(...)
vector< " " >::insert(...)
vector<" ">::push_back(...)

It looks like maybe the vector isn't valid, because _MyFirst and similar data members are showing values of 0xcdcdcdcd in the debugger. But the vector is a member variable...

Update: The vector isn't valid because it's never made. I'm getting a channel ID value stomp, which is making me treat one type of channel as another.

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Update:

Searching through with the debugger again, it appears that my method for giving each "channel" it's own, unique ID isn't giving me a unique ID:

inline static const char channel<template args>::idFunction() {
    return reinterpret_cast<char>(&channel<CHANNEL_IDENTIFY>::idFunction);
};

Update2: These two are giving the same:

slaveChannel<switchboard, ALLEGRO_BITMAP*, entityInfo<ALLEGRO_BITMAP*>
slaveChannel<key<c>, char, push<char>

Sooo, having another compiled channel type changing things makes sense, because it shifts around the values of the idFunctions? But why are there two idFunctions with the same value?

Answer 1

As a general answer (though aaa's comment alludes to this): When something like this affects whether a bug occurs, it's either because (a) you're wrong and it is being run, or (b) the way that the inclusion of that code happens to affect your code, data, and memory layout in the compiled program causes a heisenbug to change from visible to hidden.

The latter generally occurs when something involves undefined behavior. Sometimes a bogus pointer value will cause you to stomp on a bit of your code (which might or might not be important depending on the code layout), or sometimes a bogus write will stomp on a value in your data stack that might or might not be a pointer that's used later, or so forth.

As a simple example, supposing you have a stack that looks like:

float data[10];
int never_used;
int *important pointer;

And then you erroneously write

data[10] = 0;

Then, assuming that stack got allocated in linear order, you'll stomp on never_used, and the bug will be harmless. However, if you remove never_used (or change something so the compiler knows it can remove it for you -- maybe you remove a never-called function call that would use it), then it will stomp on important_pointer instead, and you'll now get a segfault when you dereference it.

Answer 2

you seem to be returning address of the function as a character? that looks weird. char has much smaller bit count than pointer, so it's highly possible you get same values. that could reason why changing code layout fixes/breaks your program