问题
As I wrote an answer to How is it possible to use pow without including cmath library I fear to have proven that missing an include of a needed header is actually undefined behavior, but since I have not found any consent of that fact I like to impose the formal question:
Is missing a required header i.e.
#include <iostream>
int main()
{
std::cout << std::pow(10, 2);
}
- Ill-formed ( [defns.ill.formed] ) code?
- Invoking undefined behavior ( [defns.undefined] )?
- If it is not 1 and 2, is it unspecified behavior [defns.unspecified] or implementation-defined behavior [defns.impl.defined]?
- If not 1. i.e. if this code is well-formed, wouldn't that contradict [using.headers] and [intro.compliance] "accept and correctly execute a well-formed program"?
As in my answer I tend to affirm both questions, but [using.headers] is very confusing because of Difference between Undefined Behavior and Ill-formed, no diagnostic message required . As [defns.well.formed] implies that a program constructed to the ODR is well formed, and there is specification of whenever the for example iostream must not define pow, one could argue this is still unspecified behavior ( [defns.unspecified]). I don't want to rely only of my standard interpretation skills for a definitive answer for such an important question. Note that the accepted i.e. the only other answer does not answer if the code is UB nor does the question asks it.
回答1:
It is unspecified whether this program is well-formed or ill-formed (with a required diagnostic, because name lookup doesn’t find pow). The possibilities arise from the statement that one C++ header may include another, which grants permission to the implementation to give this program either of just two possible interpretations.
Several similar rules (e.g., that a template must have at least one valid potential specialization) are described as rendering the program ill-formed, no diagnostic required, but in this situation that freedom is not extended to the implementation (which is arguably preferable). That said, an implementation is allowed to process an ill-formed program in an arbitrary fashion so long as it issues at least one diagnostic message, so it’s not completely unreasonable to group this situation with true undefined behavior even though the symptoms differ usefully in practice.
来源：https://stackoverflow.com/questions/61327488/is-missing-a-required-include-undefined-behavior