C ++ negative value assigned to unsigned type

1, when signed less bytes assigned to multi-byte signed, placing low byte, high-byte complement sign bit.
2, unsigned char assigned to integer variables, placing the lower byte, the upper byte of 0s;
3, when the symbol character assigned to integer variables, placing low byte, high-byte complement the sign bit.
4, a conversion is to be understood that a binary-based, then the sign bit is determined according to the type of original. General problem does not occur in the range beyond the range error will occur.

One,

#include <iostream>

int main(){
	// 将右边的数字转换为二进制再赋予
	unsigned char a1 = -1;  
	signed char a2 = -1;
	// 类型自动转换
	std::cout<<(int)a1<<" "<<(int)a2<<std::endl;
	if(a1 == a2){
		std::cout<<"ok";
	}
	return 1;
} 

It will be converted into -1 11111111 complement form, then the conversion time of the signed int type, for unsigned char, high bit 0; signed char the high bit of the sign bit. When printing in accordance with the signed int type print
output:

255 -1

--------------------------------

two,

#include <iostream>

int main(){
	unsigned char a1 = 128;
	// 数值溢出
	signed char a2 = 128;
	std::cout<<(int)a1<<" "<<(int)a2<<std::endl;
	if(a1 == a2){
		std::cout<<"ok";
	}
	return 1;
} 

128 within the range of an unsigned char, no problem. But beyond 128 signed char type, binary 10000000, assigned to this time a signed int, upper extended sign bit, becomes -128.
Output:

128 -128

--------------------------------

three,

#include <iostream>

int main(){
	unsigned char a1 = 256;
	signed char a2 = 256;
	std::cout<<(int)a1<<" "<<(int)a2<<std::endl; 
	if(a1 == a2){
		std::cout<<"ok";
	}
	return 1;
} 

Output:

0 0
ok
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