7

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

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4
  • 4
    This is a job for a static analyzer. std::string has a cont char* converting constructor so you can't stop it from accepting a null pointer. Commented Oct 9, 2019 at 12:25
  • @NathanOliver exactly (Im painfully aware of that) - do you think cpp core check can help here ? Commented Oct 9, 2019 at 12:26
  • 1
    I would hope so. They should be able to detect the null pointer dereference. Commented Oct 9, 2019 at 12:29
  • Accept nullptr or not is a pre-condition question. Commented Oct 9, 2019 at 15:00

3 Answers 3

Reset to default
4

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

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

I will try that
Last time I've checked cppcheck it suffered from lack of proper C++11 support and complained about syntax errors everywhere. Did they fix this and even implement support for C++17 in recent releases?
@VTT I think they try to be roughly up to date (no idea w.r.t C++17, but the C++11 basics should be fine), but I guess manually specifying which tests to run might be the way to go.
It seems VS2017 cpp core check is not able to detect this one
@lubgr I tried to give cppcheck a spin - what options are you using exactly ? at first it doesn't find this error - checking includes it tells me it cannot find <string> header but tells me this isn't an issue...
|
3

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;
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4 Comments

If we just could add this to std::string itself- then it would be great !
You need an overload for void test(const char*) as well (and have it call test("") if null), otherwise the caller can do something like const char *s = nullptr; test(s); and still crash at runtime.
@RemyLebeau thats a good point - I am less worried about that situation at the time being though
I ended up modifying the basic_string template ala. ` basic_string(int) = delete; basic_string(::std::nullptr_t) = delete;` - this ofc. won't catch all cases but does indeed seem to catch the direct cases at least
2

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);
         ~~~~^
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1 Comment

Even without compile-time checking for null, you should still use a trampoline, it can do something like this to ensure no exception is thrown on a null pointer: void test(const char* ptr) { if (!ptr) ptr = ""; test(std::string(ptr)); } or void test(const char* ptr) { test(std::string(ptr ? ptr : "")); }

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