constructing string from NULL?

user_2261538 0
Viewed 269

After some change of the code-base I came accross this gotcha:

#include <string>

void test(const std::string& s){
}

int main()
{
    test(NULL);
    return 0;
}

https://godbolt.org/z/7uJnef

This throws an exception. Changing to 'nullptr' helps nothing (still no error or warning).

I guess my question is, is there a way to detect or find this error at pre-runtime throughout the sourcecode ? perhaps some compiler warning, etc. (using MSVC VS-2017)

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

c++17 c++ string
3 Answers

Running cppcheck (version 1.89) on the example file yields:

Checking test.cpp ...
test.cpp:9:10: error: Null pointer dereference [nullPointer]
test(NULL);
     ^
user_95935960

You can add a couple of prohibited overloads capturing use of 0, NULL or nullptr arguments:

void test(int bad_argument) = delete;
void test(::std::nullptr_t bad_argument) = delete;
user_78606700

You can add a trampoline function that checks for NULL pointer at compile (and run) time, if your compiler supports it. For GCC it would look like this:

void test(const std::string& s){

}

void test(const char* ptr  __attribute__((nonnull))) {
    test(std::string(ptr));
}

int main()
{
    test(NULL);
    return 0;
}

The warning you get is:

<source>:13:14: warning: null passed to a callee that requires a non-null argument [-Wnonnull]

    test(NULL);
         ~~~~^
user_7166820
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