Question

According to the standard, what is the difference in behavior between declaring variables in control structures versus declaring variables elsewhere? I can’t seem to find any mention of it.

If what I’m referring to isn’t clear, here’s an example:

if (std::shared_ptr<Object> obj = objWeakPtr.lock())

As you can see, I’m declaring and initializing a local variable, obj, in the if block.

Also, is there any technical reason as to why this syntax isn’t given any special behavior when used in place of a conditional? For example, adding an additional set of brackets results in a compiler error; this also prevents the variable from being chained with other conditions.

// Extra brackets, won't compile.
if ((std::shared_ptr<Object> obj = objWeakPtr.lock()))

// If the above were valid, something like this could be desirable.
if ((std::shared_ptr<Object> obj = objWeakPtr.lock()) && obj->someCondition())

Answer 1

According to the standard, what is the difference in behavior between declaring variables in control structures versus declaring variables elsewhere? I can’t seem to find any mention of it.

Declarations inside control structure introductions are no different that declarations elsewhere. That’s why you can’t find any differences.

6.4/3 does describe some specific semantics for this, but there are no surprises:

[n3290: 6.4/3]: A name introduced by a declaration in a condition
(either introduced by the type-specifier-seq or the declarator of the
condition) is in scope from its point of declaration until the end of
the substatements controlled by the condition. If the name is
re-declared in the outermost block of a substatement controlled by the
condition, the declaration that re-declares the name is ill-formed. [..]

---

Also, is there any technical reason as to why this syntax isn’t given any special behavior when used in place of a conditional? For example, adding an additional set of brackets results in a compiler error; this also prevents the variable from being chained with other conditions.

An if condition can contain either a declarative statement or an expression. No expression may contain a declarative statement, so you can’t mix them either.

[n3290: 6.4/1]: Selection statements choose one of several flows of control.

selection-statement:
    if ( condition ) statement
    if ( condition ) statement else statement
    switch ( condition ) statement
condition:
    expression
    attribute-specifier-seq[opt] decl-specifier-seq declarator = initializer-clause
    attribute-specifier-seq[opt] decl-specifier-seq declarator braced-init-list

It all just follows from the grammar productions.