从DCL60-CPP理解不合规代码示例：遵守一定义规则

Question

I'm looking for some secure coding guideline and came across the SEI CERT C++ Coding Standard.

Most things are clear so far, but I don't understand the last Noncompliant Code Example from DCL60-CPP: Obey the one-definition rule .

In this noncompliant code example, the constant object n has internal linkage but is odr-used within f() , which has external linkage. Because f() is declared as an inline function, the definition of f() must be identical in all translation units. However, each translation unit has a unique instance of n , resulting in a violation of the ODR.

const int n = 42;

int g(const int &lhs, const int &rhs);

inline int f(int k) {
    return g(k, n);
}

I tried to put the shown code in a header file and included it in two separate cpp files. I then compiled it with clang++ and g++. Both without warnings. It executed normally.

Edit: So what I don't understand is how or under what circumstances the shown Example violates the ODR.

Answer 1

You did violate the ODR. Violating the ODR is "undefined behaviour".

One of the more common forms of undefined behaviour is "doing exactly what the programmer expected". In my experience though, an even more common form is "doing exactly what the programmer expected nearly all the time, but crashing randomly occassionally".

This particular UB will probably always work - until you switch on improved link-time optimizations, when it may all come crashing down in a heap around your head.