相关疑难解决方法(0)

我在这个网站上搜索了一个答案,发现许多对未签名/签名比较的回复,但是这个问题是只比较了无符号参数,但它仍然很有趣.

以下代码的问题在于第一个if-statment不会发生("hello"),而第二个("world")就是这样.我已经解释为在if-statment 内部进行的计算会生成一个负数但是保存到变量的结果完全相同的计算不会(即使结果被保存到有符号变量).

使用的编译器是gcc 4.4.

unsigned short u16_varHigh;  
unsigned short u16_varLow;  
unsigned short u16_Res1;  
signed short   s16_Res1;  

u16_varHigh = 0xFFFF;  
u16_varLow = 10;

u16_Res1 = u16_varLow - u16_varHigh; // response is 11 as expected  
s16_Res1 = u16_varLow - u16_varHigh; // response is 11 as expected

// Does not enter  
if( (u16_varLow - u16_varHigh) > (unsigned short)5 )  
{  
 printf( "hello" );  
}

// Does enter  
if( (unsigned short)(u16_varLow - u16_varHigh) > 5 )  
{  
 printf( "world" …

考虑以下代码:

#include <cstdint>

class A
{
public:

    explicit A(uint8_t p_a){ m_a = p_a; };
    uint8_t get_a() const {return m_a;}

private:

    uint8_t m_a;
};

int main()
{
    A a {0x21U};
    A aa{0x55U};

    uint8_t mask{a.get_a() | aa.get_a()};

    return 0;
}

当我尝试编译this(gcc 5.4.0)时,我收到以下错误:

main.cpp: In function ‘int main()’:
main.cpp:20:28: warning: narrowing conversion of ‘(int)(a.A::get_a() | aa.A::get_a())’ from ‘int’ to ‘uint8_t {aka unsigned char}’ inside { } [-Wnarrowing]
     uint8_t mask{a.get_a() | aa.get_a()};

我真的不明白为什么会有任何缩小.该int类型从未在我的代码中的任何地方使用过,所有内容都是用unsigned chars语言编写的.即使我明确地转向unsigned char我得到错误:

uint8_t …