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Editorial Team
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Asked: 2026-05-15T09:44:17+00:00

Let’s pretend my program contains a specific construct the C++ Standard states to be

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Let’s pretend my program contains a specific construct the C++ Standard states to be unspecified behavior. This basically means the implementation has to do something reasonable but is allowed not to document it. But is the implementation required to produce the same behavior every time it compiles a specific construct with unspecified behavior or is it allowed to produce different behavior in different compiles?

What about undefined behavior? Let’s pretend my program contains a construct that is UB according to the Standard. The implementation is allowed to exhibit any behavior. But can this behavior differ between compiles of the same program on the same compiler with same settings in the same environment? In other words, if I dereference a null pointer on line 78 in file X.cpp and the implementation formats the drive in such case does it mean that it will do the same after the program is recompiled?

The question is… I compile the same program with the same compier in the same environment with the same compiler settings. Will construct stated to be unspecified behavior and undefined behavior produce each the same behavior on each compile or are they allowed to differ between compiles?

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1 Answer

  1. Editorial Team

    Undefined behavior can vary between runs of the same program, and even between execution of the same code in the same run of the program. As an example, the value of an uninitialized (automatic) variable is undefined, and then its actual value is just whatever value that happened to be at that place in memory. Obviously, this can vary.

    EDIT:

    This goes for unspecified behavior too. For example, the order of evaluation of function arguments is unspecified, so if they have side effects, those side effects can occur in any order. This may print “Hi!Ho!” or “Ho!Hi!”:

    f( printf("Hi!"), printf("Ho!") );

    This can vary between executions, too. As the standard says:
    “An instance of the abstract machine can thus have more than one possible execution sequence for a given program and a given input.” The difference is that with undefined behavior, anything can happen: the computer can explode, reformat the disk, or whatever. If the behavior is unspecified, the computer is not allowed to explode.

    There is also implementation-defined behavior, such as the value of sizeof(int). This must be the same at all times, for the same compiler.

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