c++ - unsigned int 导致无限循环

Question

下面有两个循环。第一个效果很好，而第二个是无限循环。为什么？

for (unsigned int i=0; i<3; ++i)
{
    std::cout << "i= " << i << std::endl; // this gives proper result
}

for (unsigned int i=3; i>=0; --i)
{
    std::cout << "i= " << i << std::endl; // infinite loop
}

Answer 1

unsigned int 永远不能小于 0。这就是它 unsigned 的原因。如果您打开一些警告标志，您的编译器应该告诉您您的问题:i >= 0 对于 unsigned 值始终为真。

例如，Clang 根本不需要特殊标志来发出警告:

example.cpp:5:29: warning: comparison of unsigned expression >= 0 is always true
      [-Wtautological-compare]
    for (unsigned int i=3; i>=0; --i)
                           ~^ ~
1 warning generated.

GCC 需要 -Wextra:

example.cpp: In function ‘int main()’:
example.cpp:5: warning: comparison of unsigned expression >= 0 is always true