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Asked: 2026-05-27T06:51:04+00:00

Edit : I’ve restored the original title but really what I should have asked

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Edit: I’ve restored the original title but really what I should have asked was this: ‘How do C++ linkers handle class methods which have been defined in multiple object files’

Say I have a C++ class defined in a header along these lines:

class Klass
{
    int Obnoxiously_Large_Method()
    {
        //many thousands of lines of code here
    }
}

If I compile some C++ code which uses ‘Obnoxiously_Large_Method’ in several locations, will the resulting object file always inline the code for ‘Obnoxiously_Large_Method’ or will it optimise for size (for example, when using g++ -Os) and create a single instance of ‘Obnoxiously_Large_Method’ and use it like a normal function?, if so, how do linkers resolve the collisions between other object files which have instantiated the same function?. Is there some arcane C++ namespace Juju which keeps the separate object instances of method from colliding with each other?

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1 Answer

  1. Editorial Team

    7.1.2 Function specifiers

    A function declaration (8.3.5, 9.3, 11.4) with an inline specifier
    declares an inline function. The inline specifier indicates to the
    implementation that inline substitution of the function body at the
    point of call is to be preferred to the usual function call mechanism.
    An implementation is not required to perform this inline substitution
    at the point of call; however, even if this inline substitution is
    omitted, the other rules for inline functions defined by 7.1.2 shall
    still be respected.

    So, the compiler is not required to actually ‘inline’ any function.

    However, the standard also says,

    An inline function with external linkage shall have the same address in all translation units.

    Member functions normally have external linkage (one exception is when the member function belongs to a ‘local’ class), so inline functions must have a unique address for cases where the address of the function is taken. In this case, the compiler will arrange for the linker to throw away all but one instance of a non-inlined copy of the function and fix-up all address references to the function to be to the one that’s kept.

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