The contents of the bits used as padding are indeterminate. When applied to an operand that has struct type, the result is the total number of bytes in a variable of that type, including any padding. A local variable declared by a fixed statement is considered read-only.
It is the programmer's responsibility to ensure that pointers created by fixed statements do not survive beyond execution of those statements. For example, when pointers created by fixed statements are passed to external APIs, it is the programmer's responsibility to ensure that the APIs retain no memory of these pointers.
Fixed objects may cause fragmentation of the heap because they can't be moved. For that reason, objects should be fixed only when absolutely necessary and then only for the shortest amount of time possible. The first statement fixes and obtains the address of a static field, the second statement fixes and obtains the address of an instance field, and the third statement fixes and obtains the address of an array element. The fourth fixed statement in the example above produces a similar result to the third.
In an unsafe context array elements of single-dimensional arrays are stored in increasing index order, starting with index 0 and ending with index Length - 1. For multi-dimensional arrays, array elements are stored such that the indices of the rightmost dimension are increased first, then the next left dimension, and so on to the left. Length - 1 represent addresses of the elements in the array. Likewise, the variables ranging from p[0] to p[a.
Length - 1] represent the actual array elements. Given the way in which arrays are stored, we can treat an array of any dimension as though it were linear. Modifying objects of managed type through fixed pointers can results in undefined behavior. For example, because strings are immutable, it is the programmer's responsibility to ensure that the characters referenced by a pointer to a fixed string are not modified.
The automatic null-termination of strings is particularly convenient when calling external APIs that expect "C-style" strings. Note, however, that a string instance is permitted to contain null characters.
Fixed size buffers are used to declare "C style" in-line arrays as members of structs, and are primarily useful for interfacing with unmanaged APIs. A fixed size buffer is a member that represents storage for a fixed length buffer of variables of a given type. A fixed size buffer declaration introduces one or more fixed size buffers of a given element type. Fixed size buffers are only permitted in struct declarations and can only occur in unsafe contexts Unsafe contexts.
A fixed size buffer declaration may include a set of attributes Attributes , a new modifier Modifiers , a valid combination of the four access modifiers Type parameters and constraints and an unsafe modifier Unsafe contexts.
The attributes and modifiers apply to all of the members declared by the fixed size buffer declaration. It is an error for the same modifier to appear multiple times in a fixed size buffer declaration. The buffer element type of a fixed size buffer declaration specifies the element type of the buffer s introduced by the declaration.
The buffer element type must be one of the predefined types sbyte , byte , short , ushort , int , uint , long , ulong , char , float , double , or bool. The buffer element type is followed by a list of fixed size buffer declarators, each of which introduces a new member. A fixed size buffer declarator consists of an identifier that names the member, followed by a constant expression enclosed in [ and ] tokens. The constant expression denotes the number of elements in the member introduced by that fixed size buffer declarator.
The type of the constant expression must be implicitly convertible to type int , and the value must be a non-zero positive integer. A fixed size buffer declaration that declares multiple fixed size buffers is equivalent to multiple declarations of a single fixed size buffer declaration with the same attributes, and element types. Member lookup Operators of a fixed size buffer member proceeds exactly like member lookup of a field. When a fixed size buffer member is referenced as a simple name, the effect is the same as a member access of the form this.
I , where I is the fixed size buffer member. In a member access of the form E. I , if E is of a struct type and a member lookup of I in that struct type identifies a fixed size member, then E. I is evaluated an classified as follows:. The subsequent elements of the fixed size buffer can be accessed using pointer operations from the first element. Unlike access to arrays, access to the elements of a fixed size buffer is an unsafe operation and is not range checked.
Fixed size buffers are not subject to definite assignment checking Definite assignment , and fixed size buffer members are ignored for purposes of definite assignment checking of struct type variables. When the outermost containing struct variable of a fixed size buffer member is a static variable, an instance variable of a class instance, or an array element, the elements of the fixed size buffer are automatically initialized to their default values Default values.
In all other cases, the initial content of a fixed size buffer is undefined. In an unsafe context, a local variable declaration Local variable declarations may include a stack allocation initializer which allocates memory from the call stack. Taken together, these specify the required allocation size. A stack allocation initializer of the form stackalloc T[E] requires T to be an unmanaged type Pointer types and E to be an expression of type int.
If E is a negative value, then the behavior is undefined. If E is zero, then no allocation is made, and the pointer returned is implementation-defined. If there is not enough memory available to allocate a block of the given size, a System. StackOverflowException is thrown.
Stack allocation initializers are not permitted in catch or finally blocks The try statement. There is no way to explicitly free memory allocated using stackalloc.
All stack allocated memory blocks created during the execution of a function member are automatically discarded when that function member returns.
As already said it is an operator not a function, but additionally it is one of the operators for which operator overloading is not allowed:. I can think of no conceivable reason why you would want to overload this in any case. If you have an class for which size information other than that which sizeof yields is required, then simply add a member function to provide that information; as for example in std::string:size which returns the length of the string managed by the object rather than the size of the object which is semantically different; you do not want to monkey with the semantics of sizeof!
You could write your own type traits that will work similar to built-in sizeof operator. But this have no sense since only creators of the compiler are knows actual values of value for the implementation. The compiler works out the type of the argument unless it's already a type , then substitutes the expression with an integer constant. I saw this post when searching for a way to get the same functionality as the sizeof operator.
I implemented a global template function like this:. Without the keyword constexpr, the function will compile still. With the use of constexpr, the whole function evaluates to a constant, which in my knowledge should be exactly equivalent to how the sizeof operator works? Correct me if I am wrong. In use I call the function like this, with an added parantesis after the type:. The source will also show how sizeof can be implemented. So, I think, it is impossible to write sizeof - complain function for dynamically allocated buffer.
It's an operator in C. We can implement its functionality something like follows. Asked By: dato datuashvili. You can't use it where such a value is required - for example when initializing static variables, unless a compiler specific extension allows it the C Standard allows an implementation to have extensions to what it treats as constant.
Are you perhaps interested in implementing bounds checking? If so, there are a number of different ways to go about that. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams?
Collectives on Stack Overflow. Learn more. How Does sizeof Array work Ask Question. Asked 12 years, 7 months ago. Active 5 years, 3 months ago. Viewed 26k times. Improve this question. Kazoom Kazoom 5, 16 16 gold badges 54 54 silver badges 67 67 bronze badges.
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Johannes Schaub - litb Johannes Schaub - litb k gold badges silver badges bronze badges. Good that you mention C99 VLAs.
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