compilation error when Base Constructor calling pure virtual method
in [C++]
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From: Daniel Krügler on 6 Jul 2010 21:09 On 7 Jul., 04:18, liam_herron <liam_her...(a)hotmail.com> wrote: > Why does this compile when I define the body of the constructor in the > header versus when I define the body of the constructor in the cpp file? > > // Works > > class A > { > public: > A() {f();} > virtual ~A() {} > virtual void f() = 0; > int a1_; > }; What do you mean with "works" here? Invoking a pure virtual function in the constructor or destructor causes undefined behavior. Many (most) implementations will diagnose that, but that is not required. Note that this category applies, even, if this pure function does have a definition. > // Generates Linker Error > class A2 > { > public: > A2(); > virtual ~A2() {} > virtual void f() = 0; > int a2_; > }; The linker error may have different reasons, it usually is an implementation-specific outcome of something the standard declares as causing undefined behaviour. In this example the reason could simply be because you invoked the pure virtual function in the constructor or because you did not compile the translation unit that contains the constructor definition. HTH & Greetings from Bremen, Daniel Kr�gler -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Chris Uzdavinis on 7 Jul 2010 05:17
On Jul 6, 9:18 pm, liam_herron <liam_her...(a)hotmail.com> wrote: > Why does this compile when I define the body of the constructor in the > header versus > when I define the body of the constructor in the cpp file? [snip code showing ctor invoking pure virtual function, inline and out-of-line] Defining the constructor in the class declaration causes (requests) it to be "inlined", such that code for the function doesn't normally get inserted into your compiler's object code except at the point where it's called--but it's not called from anywhere in your example and so no code is generated; the linker never sees it. (Of course, if the compiler opts to not inline the ctor afterall, then you would see the linker error in both of your examples.) -- Chris [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] |