C++ 在矢量存取中未定义的行为
原标题：C++ Undefined behaviour in vector access
Following up the p0532r0.pdf documentation the following piece of code results in UB
#include
#include
struct X {
const int i;
X(int _i) : i(_i) {}
friend std::ostream& operator<< (std::ostream& os, const X& x) {
return os << x.i;
};
};
int main(){
std::vector v;
v.push_back(X(42));
v.clear();
v.push_back(X(77));
std::cout << v[0]; // undefined behaviour
}
However no UB is detected when the above piece of code is run with sanitiser. demo Why does compiler not detect this error? Apart from that based on the P0532R0, when type of vector is a class that has a const member , the internal usage of placement new causes UB. How is this issue overcome ?
问题回答
Firstly, this behaviour is undefined by the standard, but is implicitly well-defined on GCC. From your linked PDF:
AFAIK, gcc currently does not optimize the constant/reference case, but does optimizations for the vptr part.
So in a sense there s nothing to detect.
Secondly, the reason this UB is not detected by ubsan: it doesn t check for it at all in the first place.
The manual (for 9.5) lists the behaviours checked by ubsan here, and links the equivalent clang ubsan documentation.
Neither claim to detect the particular un-laundered access to a placement-new overwritten object with a const member that you re concerned with.
BTW, Your demo is using GCC 9.1. None of this changes with later versions up to and including trunk AFAICS, but that s why I linked the 9.5 manual.
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