C++ – Difference between an inline function and static inline function

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Can anybody tell me what the difference is between an inline function and static inline function?

In which cases should I prefer static inline over inline?

I am asking this question because I have an inline function for which I am facing compilation issues during linking (relocation error:... symbol has been discarded with discarded section ...). I made it a normal function and it worked.
Now some of my seniors told me try with static inline.
Below is my function:

inline void wizSendNotifier (const char* nn_name, bpDU* arg=0, int aspect = -1)
{
   wizuiNotifier* notifier = ::wizNtrKit.getNotifier (nn_name);
   notifier->notify (arg, aspect);
}

and this not inside a class. This is inside a header file!

I guess the call to a static function should be done only in the particular TU where it is defined.

Since my function is in a header file and if i make it static, will it be the case that where ever I include that header file the static function can used used in that translation unit?

Best Answer

The non-static inline function declaration refers to the same function in every translation unit (source file) that uses it.

The One Definition Rule requires that the body of the function definition is identical in every TU that contains it, with a longish definition of "identical". This is usually satisfied provided that the source files all use the same header, and provided that the function doesn't use any global names with internal linkage (including static functions) or any macros that are defined differently in different TUs.

I don't remember encountering that particular linker error before, but it's at least possible that one of these restrictions is responsible. It's your responsibility to satisfy the requirements: undefined behavior with no diagnostic required if you don't.

The static inline function declaration refers to a different function in each translation unit, that just so happens to have the same name. It can use static global names or macros that are different in different TUs, in which case the function might behave differently in the different TUs, even though its definition in the header file "looks the same".

Because of this difference, if the function contains any static local variables then it behaves differently according to whether it is static or not. If it is static then each TU has its own version of the function and hence its own copy of the static local variables. If it's inline only, then there is only one copy of the static local variables used by all TUs.

Related Solutions

C++ – the difference between #include and #include “filename”

In practice, the difference is in the location where the preprocessor searches for the included file.

For #include <filename> the preprocessor searches in an implementation dependent manner, normally in search directories pre-designated by the compiler/IDE. This method is normally used to include standard library header files.

For #include "filename" the preprocessor searches first in the same directory as the file containing the directive, and then follows the search path used for the #include <filename> form. This method is normally used to include programmer-defined header files.

A more complete description is available in the GCC documentation on search paths.

C++ – What are the differences between a pointer variable and a reference variable in C++

A pointer can be re-assigned:

int x = 5;
int y = 6;
int *p;
p = &x;
p = &y;
*p = 10;
assert(x == 5);
assert(y == 10);

A reference cannot be re-bound, and must be bound at initialization:

int x = 5;
int y = 6;
int &q; // error
int &r = x;

A pointer variable has its own identity: a distinct, visible memory address that can be taken with the unary & operator and a certain amount of space that can be measured with the sizeof operator. Using those operators on a reference returns a value corresponding to whatever the reference is bound to; the reference’s own address and size are invisible. Since the reference assumes the identity of the original variable in this way, it is convenient to think of a reference as another name for the same variable.
```
int x = 0;
int &r = x;
int *p = &x;
int *p2 = &r;

assert(p == p2); // &x == &r
assert(&p != &p2);
```

You can have arbitrarily nested pointers to pointers offering extra levels of indirection. References only offer one level of indirection.

int x = 0;
int y = 0;
int *p = &x;
int *q = &y;
int **pp = &p;

**pp = 2;
pp = &q; // *pp is now q
**pp = 4;

assert(y == 4);
assert(x == 2);

A pointer can be assigned nullptr, whereas a reference must be bound to an existing object. If you try hard enough, you can bind a reference to nullptr, but this is undefined and will not behave consistently.

/* the code below is undefined; your compiler may optimise it
 * differently, emit warnings, or outright refuse to compile it */

int &r = *static_cast<int *>(nullptr);

// prints "null" under GCC 10
std::cout
    << (&r != nullptr
        ? "not null" : "null")
    << std::endl;

bool f(int &r) { return &r != nullptr; }

// prints "not null" under GCC 10
std::cout
    << (f(*static_cast<int *>(nullptr))
        ? "not null" : "null")
    << std::endl;

You can, however, have a reference to a pointer whose value is nullptr.

Pointers can iterate over an array; you can use ++ to go to the next item that a pointer is pointing to, and + 4 to go to the 5th element. This is no matter what size the object is that the pointer points to.
A pointer needs to be dereferenced with * to access the memory location it points to, whereas a reference can be used directly. A pointer to a class/struct uses -> to access its members whereas a reference uses a ..
References cannot be put into an array, whereas pointers can be (Mentioned by user @litb)
Const references can be bound to temporaries. Pointers cannot (not without some indirection):
```
const int &x = int(12); // legal C++
int *y = &int(12); // illegal to take the address of a temporary.
```
This makes const & more convenient to use in argument lists and so forth.

Best Answer

Related Solutions

C++ – the difference between #include and #include “filename”

C++ – What are the differences between a pointer variable and a reference variable in C++

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