new(std::nothrow) int[n] throws an exception

Question

#include <iostream>
#include <new>

int main()
{
    int n = -1;
    try
    {
        int *p = new(std::nothrow) int[n];
        if(!p)
            std::cout << "new expression returned nullptr\n";
    }
    catch(const std::bad_array_new_length& e)
    {
        std::cout << "new expression threw " << e.what() << std::endl;
    }
}

Why does this code throw an exception? It prints new expression threw std::bad_array_new_length. According to the standard the new expression should return nullptr in this case.

If the expression in a noptr-new-declarator is present, it is implicitly converted to std::size_t. The expression is erroneous if:

— the expression is of non-class type and its value before converting to std::size_t is less than zero;

[...]

If the expression is erroneous after converting to std::size_t:

— if the expression is a core constant expression, the program is ill-formed;

— otherwise, an allocation function is not called; instead

— if the allocation function that would have been called has a non-throwing exception specification (14.5), the value of the new-expression is the null pointer value of the required result type;

— otherwise, the new-expression terminates by throwing an exception of a type that would match a handler (14.4) of type std::bad_array_new_length (17.6.3.2).

Compiled with gcc 9.2

Filed [gcc bug 93016](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93016). — Barry, Dec 19 '19 at 18:10
Hm. As an aside, "after converting to std::size_t" looks erroneous. Perhaps an editing error of the standards committee? — Deduplicator, Dec 19 '19 at 18:14
@T.C. Should it throw or it's a bug? I didn't understand this from the text. — Lassie, Dec 19 '19 at 20:19
@Lassie The core issue shows throwing that exception was once a bug in the standard. — Deduplicator, Dec 20 '19 at 12:58

score 1 · Answer 1 · answered Dec 19 '19 at 18:05

1

I suspect that this is a bug in libstdc++; running this code using clang and libc++ prints "new expression returned nullptr"

answered Dec 19 '19 at 18:05

Marshall Clow

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An error of libstdc++? Hardly, that's in the compiler proper. – Deduplicator Dec 19 '19 at 18:16
@Deduplicator - I don't think so. take a look at [the libc++ implementation](https://github.com/llvm/llvm-project/blob/master/libcxx/src/new.cpp#L112) – Marshall Clow Dec 19 '19 at 18:20
That shows that their nothrow op-new function properly swallows all exceptions. But one can no longer check for exceeded implementation-limit or negative input by then, that train has long ago left the station. Thus, it is irrelevant here. – Deduplicator Dec 19 '19 at 18:26
1

Yeah, this isn't a library issue. – T.C. Dec 19 '19 at 20:05

new(std::nothrow) int[n] throws an exception

1 Answers1