c++ double initialization- default values?

Question

I'm debugging some C++ code, and have come across a double that never appears to have been given a value. It is declared with the line

double x;

Having not used C or C++ much before, I'm unsure whether doubles have a value that they default to? I.e. Given the declaration above, if x is never specifically given a value, will it automatically be 0, or will it be null?

The x is used in a calculation elsewhere in the code, and the result of the calculation is meant to be displayed to the user - at the moment it's not... The calculation is:

y * asin(sin(x / y) * sin(a * b));

I would assume that if x defaults to 0, this would cause a compile/runtime error? If x defaults to 0, surely the calculation would return 0, and 0 would be displayed to the user?

declarations of type `double x;` do not assign default value to x (what exactly is default value anyway ?) if the declaration is inside a function's scope ! you must write it yourself `double x = 0.0;` — Raxvan, Sep 18 '14 at 08:57
http://en.cppreference.com/w/cpp/language/default_initialization — EdChum, Sep 18 '14 at 08:58

Some programmer dude · Accepted Answer · 2022-07-12T09:00:45.487

13

It depends on where the variable is declared.

If it's declared as a global variable then it will be zero-initialized before main starts running.

If it's declared as a non-static local variable inside a function then its value is indeterminate (in reality it will be whatever is in the memory occupied by the variable, it will be seemingly random).

Using uninitialized (non-static) local variables leads to undefined behavior.

If declared as a static local variable, then it will also be zero-initialized in the first call to the function.

edited Jul 12 '22 at 09:00

answered Sep 18 '14 at 08:59

Some programmer dude

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what if it's a member of a class? – athos Feb 04 '17 at 09:56
@athos Then it depends on where the containing object were declared, and what initialization the class does to the member. – Some programmer dude Feb 04 '17 at 11:09

score 7 · Answer 2 · answered Sep 18 '14 at 08:57

7

If you don't specify an initialization value then the value for double x is undefined if it's on the heap/stack. If it's a global variable it will be set to 0.

answered Sep 18 '14 at 08:57

Sean

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score 1 · Answer 3 · edited Jun 20 '20 at 09:12

Zero initialization is performed in the following situations:

For every named variable with static or thread-local storage duration that is not subject to constant initialization (since C++14), before any other initialization.

As part of value-initialization sequence for non-class types and for members of value-initialized class types that have no constructors, including value initialization of elements of aggregates for which no initializers are provided.

When an array of any character type is initialized with a string literal that is too short, the remainder of the array is zero-initialized.

https://en.cppreference.com/w/cpp/language/default_initialization

https://en.cppreference.com/w/cpp/language/zero_initialization

So double data members in classes with no constructors will be initialized to zero.

*"in classes"* Only if the whole class is value-initialized. — HolyBlackCat, Sep 28 '22 at 10:53

c++ double initialization- default values?

3 Answers3