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namespace MyNamespace {
int ilength;
}
 
namespace MyNamespace {
int iwidth;
}
 
void f() {
  MyNamespace::ilength = MyNamespace::iwidth = 12;
}

The standard library introduces the namespace std for standards-provided declarations such as std::string, std::vector, and std::for_each. However, it is undefined behavior to introduce new declarations in namespace std except under special circumstances. The C++ Standard, [namespace.std], paragraphs 1 and 2 [ISO/IEC 14882-2014], states the following:

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Altering the standard namespace can cause undefined behavior in the C++ standard library.

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