CODEWITHSUNDEEP
c++staticscope
q126y
q126y

Reputation: 1671

When does static local variable comes into existence?

The book Object oriented programming in c++ by Robert Lafore says,

A static local variable has the visibility of an automatic local variable (that is, inside the function containing it). However, its lifetime is the same as that of a global variable, except that it doesn’t come into existence until the first call to the function containing it. Thereafter it remains in existence for the life of the program

What does coming into existence after first call of function mean? The storage for static local is allocated at the time program is loaded in the memory.

Upvotes: 3

Views: 456

Answers (4)

Jerry Coffin
Jerry Coffin

Reputation: 490058

The storage is allocated before main is entered, but (for example) if the static object has a ctor with side effects, those side effects might be delayed until just before the first time the function is called.

Note, however, that this is not necessarily the case. Constant initialization is only required to happen before that block is entered (not necessarily just as execution "crosses" that definition). Likewise, implementations are allowed to initialize other block-scope static variables earlier than required under some circumstances (if you want to get into the gory details of the circumstances, you can look at [basic.start.init] and [stmt.dcl], but it basically comes down to: as long as it doesn't affect the value with which it's initialized. For example, if you had something like:

    int i;
    std::cin >> i;
    { 
        static int x = i;

...the implementation wouldn't be able to initialize x until the block was entered, because the value with which it was being initialized wouldn't be known until them. On the other hand, if you had:

{ 
     static int i = 0;

...the implementation could carry out the initialization as early as it wished (and most would/will basically carry out such an initialization at compile time, so it won't involve executing any instructions at run-time at all). Even for less trivial cases, however, earlier initialization is allowed when logically possible (e.g., the value isn't coming from previous execution).

Upvotes: 4

AnT stands with Russia
AnT stands with Russia

Reputation: 320381

In C++ storage duration of an object (when raw memory gets allocated for it) and lifetime of an object are two separate concepts. The author was apparently referring to the latter one when he was talking about object's "coming into existence".

In general case it is not enough to allocate storage for an object to make it "come into existence". Lifetime of an object with non-trivial initialization begins once its initialization is complete. For example, an object of a class with a non-trivial constructor does not officially "live" until its constructor has completed execution.

Initialization of a static local object is performed when the control passes over the declaration for the very first time. Before that the object does not officially exist, even if the memory for it is already allocated.

Note that the author is not painstakingly precise in his description. It is not sufficient to just call the function containing the declaration. The control has to pass through the declaration of the object for it to begin its lifetime. If the function contains branching, this does not necessarily happen during the very first call to the function.

For object with trivial initialization (like int objects), there's no difference between storage duration and lifetime. For such objects allocating memory is all that needs to be done. But in general case allocating memory alone is not sufficient.

Upvotes: 2

Ely
Ely

Reputation: 11152

As soon as the function, which contains a static local variable, is called the static local variable is initialized.

Upvotes: 1

Alex Lop.
Alex Lop.

Reputation: 6875

It means that the static variable inside a function doesn't get initialized (by the constructor or the assignment operator) until the first call for that function.

Upvotes: 1

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