CODEWITHSUNDEEP
c++c++11
Gabriel
Gabriel

Reputation: 9432

conversion to static_cast<unsigned char>

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)

Upvotes: 1

Views: 439

Answers (2)

Serge Ballesta
Serge Ballesta

Reputation: 148890

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.

Upvotes: 3

Maxim Egorushkin
Maxim Egorushkin

Reputation: 136246

On most modern architectures char and unsigned char have the same size and alignment, hence unsigned char can represent all non-negative values of char and casting one to another does not require any CPU instructions.

Upvotes: 0

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