转换为static_cast <unsigned char>

Question

Does a conversion like:

int a[3];
char i=1;
a[ static_cast<unsigned char>(i) ];

introduce any overhead like conversions or can the compiler optimize everything away? I am interested because I want to get rid of -Wchar-subscripts warnings, but want to use a char as index (other reasons)

Answer 1

I did one test on Clang 3.4.1 for this code :

int ival(signed char c) {
    int a[] = {0,1,2,3,4,5,6,7,8,9};
    unsigned char u = static_cast<unsigned char>(c);
    return a[u];
}

Here is the relevant part or the assembly file generated with c++ -S -O3

_Z4ivala:                               # @_Z4ivala
# BB#0:
    pushl   %ebp
    movl    %esp, %ebp
    movzbl  8(%ebp), %eax
    movl    .L_ZZ4ivalaE1a(,%eax,4), %eax
    popl    %ebp
    ret

There is no trace of the conversion.

Answer 2

在大多数现代建筑char和unsigned char具有相同的大小和排列，因此unsigned char可以代表所有非负值char和铸造彼此不需要任何CPU指令。