Don't declare a variable until you have a value to initialize it with

Reason

Readability. Limit the scope in which a variable can be used. Don't risk used-before-set. Initialization is often more efficient than assignment.

Example, bad

string s;
// ... no use of s here ...
s = "what a waste";

Example, bad

SomeLargeType var;  // Hard-to-read CaMeLcAsEvArIaBlE

if (cond)   // some non-trivial condition
    Set(&var);
else if (cond2 || !cond3) {
    var = Set2(3.14);
}
else {
    var = 0;
    for (auto& e : something)
        var += e;
}

// use var; that this isn't done too early can be enforced statically with only control flow

This would be fine if there was a default initialization for SomeLargeType that wasn't too expensive. Otherwise, a programmer might very well wonder if every possible path through the maze of conditions has been covered. If not, we have a "use before set" bug. This is a maintenance trap.

For initializers of moderate complexity, including for const variables, consider using a lambda to express the initializer; see ES.28.

Enforcement

• Flag declarations with default initialization that are assigned to before they are first read.
• Flag any complicated computation after an uninitialized variable and before its use.