奇怪的溢出分配2*1024*1024*1024

html5 • • 问答 

void main()
{
    long long ll = 2 * 1024 * 1024 * 1024; 
    unsigned long long ull = (2 * 1024 * 1024 * 1024);
    std::cout << ll << "n" << ull;
}

我使用 Visual Studio 2019 的输出是

 -2147483648
18446744071562067968

我不知道为什么这里会发生溢出，请帮忙

回答

In both cases the calculation is done using integers because all the values on the right are integer literals. 2 * 1024 * 1024 * 1024 is 2,147,483,648 which 1 larger than the max 32 bit int so this part of the calculation is overflowing. To fix do the first calculation as long long using long long ll = 2LL * 1024 * 1024 * 1024; and the second calculation as an unsigned long long using unsigned long long ull = 2ULL * 1024 * 1024 * 1024;

You can learn more about integer literals here: https://en.cppreference.com/w/cpp/language/integer_literal

Here is the fixed code:

#include <iostream>
#include <limits>

using namespace std;

int main()
{
    long long ll = 2LL * 1024 * 1024 * 1024; 
    unsigned long long ull = 2ULL * 1024 * 1024 * 1024;
    std::cout << ll << "n" << ull;
}

I put an online version of this fixed code here:
https://ideone.com/5QkEls

  • Makes more sense to _lead_ the multiplication chain with `LL` so `2LL * 1024 * 1024 * 1024`. Now the product is correct even if `int` was 16-bit.
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