CODEWITHSUNDEEP
c++integer-overflow
bricklore
bricklore

Reputation: 4165

Overflow expected but adding two bytes does not

Update

Can anyone explain the reason for promoting uint8_t to int? What are the use cases for this behavior? And can I prevent the promoting?

I have these very simple code snippets which add two integer types together and print the result. I have compiled those without any optimization with g++ --std=c++11 main.cpp -o test

int main()
{
    uint8_t a = 200, b = 100;
    uint16_t result = (a + b);

    printf("result is %d\n", result);
}

Outputs:

result is 300

I would have expected (a + b) to evaluate to 44 because they are both of an 8-bit type and should overflow. But instead I get the unexpected result of 300.

If I re-run the same test with uint32_t and uint64_t, it is overflowing as expected:

int main()
{
    uint32_t a = UINT_MAX, b = UINT_MAX;
    uint64_t result = (a + b);

    printf("result is %ld\n", result);
}

Outputs:

result is 4294967294

Now I can't tell why uint8_t is treated differently to uint32_t.

Upvotes: 2

Views: 155

Answers (2)

Slava
Slava

Reputation: 44278

C++ Standard: 5.10

— Otherwise, the integral promotions (4.5) shall be performed on both operands. Then the following rules shall be applied to the promoted operands: — If both operands have the same type, no further conversion is needed.

4.5.1 Integral promotions

A prvalue of an integer type other than bool, char16_t, char32_t, or wchar_t whose integer conversion rank (4.13) is less than the rank of int can be converted to a prvalue of type int if int can represent all the values of the source type; otherwise, the source prvalue can be converted to a prvalue of type unsigned int.

This is most probably the reason:

3.9.1 Fundamental types

Plain ints have the natural size suggested by the architecture of the execution environment; the other signed integer types are provided to meet special needs.

Upvotes: 1

pm100
pm100

Reputation: 50210

integral types smaller than int are promoted to int when operations are performed on them

Upvotes: 8

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