Preferred way of conditionally initializing a const

Question

What is the preferred way to initialize a const automatic variable? I can think of several.

if statement(note: later I learned this is UB):

const std::string s;
if( condition ) { const_cast<std::string&>(s) = "first"; }
else { const_cast<std::string&>(s) = "second"; }

?: operator:

const std::string s = ( condition ) ? "first" : second;

immediately invoked function expression:

const std::string s = [ & ] () 
    { if( condition ) return "first" else return "second" } ();

Answer 1

C++ Core Guidelines advise to use lambdas for complex initialization, especially of const variables.

Answer 2

Your if statement example does not initialize any const, as s is not const.

Regarding other options (this is mainly subjective), here are my guidelines:

• Use the ?: (ternary operator) when the expression is short and easy to read. In your case, I think it would be fine.

• Use a IIFE (immediately invoked function expression) or refactor the initialization to a function which returns a value when the initialization logic is long and complex and when it would negatively impact readability of the function where the variable is initialized.