General

Many tips on the C wiki page also apply to C++
Read tips for golfing in C++ on the codegolf stack exchange.
A list of builtins is available here.

`2:1` text packing

Using utf16 strings, you can compress prints that output constant strings, or prints that use constant strings as arguments, to take half the characters.

Spoiler

Compressor

#import<cstring>
#import<clocale>
char str[]="STRING";
int i;
int main(){
	for(setlocale(LC_ALL, "en_US.UTF-8");i<strlen(str);i+=2)
		__builtin_printf("%C",str[i]+(str[i+1]<<8));
}

Decompressor

Note: compared to just printing via __builtin_printf("..."), the string must be over 13 characters to get a benefit. However, if your print was already of the form __builtin_printf("...%s",...,"..."), your string only needs to be over 3 characters to get a benefit.

int main(){__builtin_printf("%s",u"...");}

Output

If you are familiar with or translating from C:

__builtin_printf can be used in place of printf without including any header
You can #import<ios> to include common C I/O functions.
If you have several imports, you may be able to replace all of them with #import<bits/stdc++.h>, which includes everything listed here.