Resolve std::string to something else than ::std::string - is it possible? - c++

Question

This is a follow-up question from: constructing string from NULL?
The following:
void test(const std::string& s);
int main(){
test(NULL);
}
Fails when run, but is legal c++.
In order to try to catch some of those cases, as an alternative ive considering if std::string can be replaced in the following way:
#include <string>
namespace {
namespace std {
struct string : public ::std::string { //so far everything is good
};
}
}
int main ()
{
std::string hello;//failure: ambiguous symbol
return 0;
}
Gives the following error:
<source>(17): error C2872: 'std': ambiguous symbol
C:/data/msvc/14.22.27905/include\string(19): note: could be 'std'
<source>(7): note: or '`anonymous-namespace'::std'
<source>(17): error C2872: 'std': ambiguous symbol
C:/data/msvc/14.22.27905/include\string(19): note: could be 'std'
<source>(7): note: or '`anonymous-namespace'::std'
Compiler returned: 2
I guess it is not possible to make it resolve without writing a more (fully) qualified name ? but is it possible to write std::string in the global namespace and have it resolve to something else while ::std::string is a valid type.
Background: after some failed attempts with cppcheck and cpp core check im trying to find all cases of std::string str = 0 or NULL or nullptr - since those will fail at runtime. And I thought this might be a way forward.
I ended up modifying the basic_string template ala. basic_string(int) = delete;
basic_string(::std::nullptr_t) = delete; - this won't catch all cases but does indeed seem to catch the direct cases at least

Answer 1

Resolve std::string to something else than ::std::string - is it possible?
[...]
...as an alternative ive considering if std::string can be replaced in the following way...
As far as I know, it is not possible outside of your anonymous namespace scope because you have no way to resolve the ambiguity (not to my knowledge).
As you can see below, since you are inside the scope of the anonymous namespace, it will be fine:
#include <string>
namespace
{
namespace std
{
struct string : public ::std::string
{
};
}
std::string hello; // Fine
}
int main()
{
std::string hello2; // Cannot be something else that ambiguous
return 0;
}
But even worse, the problem is not about std::string itself but about the std namespace.
Indeed, outside of the scope of your anonymous namespace, every call of std becomes ambiguous too.This is the error pointed by the compiler. There are two std namespaces accessible as is from the global scope.
So the following example becomes broken:
#include <string>
#include <vector>
namespace
{
namespace std
{
struct string : public ::std::string
{
};
}
std::string hello; // Fine
}
int main()
{
std::vector<int> a; // FAIL: reference to 'std' is ambiguous
return 0;
}
To fix this ambiguity over accessing the original std namespace, you'll need to write it as follows:
::std::vector<int> a; // Fully qualified name: Only way to refer to the `::std` namespace
As you can see, it still does not solve the issue and even worse, it adds a huge inconvenience.
Therefore the morality is:
Do not hide an already existing type but create a distinct one instead.
In the same way, do not hide a namespace (by defining the same namespace into an anonymous one --> evil).
(I'm open to any improvement proposal of this answer)

Answer 2

Could you use this kind of solution : Rather than working on std::string, you work on the function that will cause the problem
#include <string>
void test(const std::string& s){
}
// add a new function which use a pointer :
void test (const char* _Nonnull s) {
test(std::string(s));
}
int main()
{
test(NULL);
return 0;
}
Then clang will generate a warning :
1 warning generated.
ASM generation compiler returned: 0
<source>:13:14: warning: null passed to a callee that requires a non-null argument [-Wnonnull]
test(NULL);
see https://godbolt.org/z/PujFor