File Scope and Global Scope: C & C++

问题

I am a student and I am confused about global and file scope variables in C and C++. Is there is any difference in both perspectives? If yes, please explain in detail.

回答1:

A variable with file scope can be accessed by any function or block within a single file. To declare a file scoped variable, simply declare a variable outside of a block (same as a global variable) but use the static keyword.

static int nValue; // file scoped variable
float fValue; // global variable

int main()
{
    double dValue; // local variable
}

File scoped variables act exactly like global variables, except their use is restricted to the file in which they are declared.

回答2:

A name has file scope if the identifier's declaration appears outside of any block. A name with file scope and internal linkage is visible from the point where it is declared to the end of the translation unit.

Global scope or global namespace scope is the outermost namespace scope of a program, in which objects, functions, types and templates can be defined. A name has global namespace scope if the identifier's declaration appears outside of all blocks, namespaces, and classes.

Example:

static int nValue; // file scoped variable
float fValue; // global variable

int main()
{
    double dValue; // local variable
}

回答3:

It is perhaps clearer to illustrate file (translation unit)-scope vs global scope when there are actually multiple translation units...

Take 2 files (each being it's own translation unit, since they don't include each other)

other.cpp

float global_var = 1.0f;
static float static_var = 2.0f;

main.cpp

#include <cstdio>

extern float global_var;
//extern float static_var; // compilation error - undefined reference to 'static_var'

int main(int argc, char** argv)
{
    printf("%f\n", global_var);
}

Hence the difference is clear.

回答4:

File scope: Any name declared outside all blocks or classes has file scope. It is accessible anywhere in the translation unit after its declaration. Names with file scope that do not declare static objects are often called global names.

In C++, file scope is also known as namespace scope.

回答5:

Read this carefully now.

You use those #include<'...'.h> statements at the top of your program/code. What you actually are doing there is telling the computer to refer to the functions prewritten in those *h*eader files.That is, those functions have file scope.You donot write the code of printf scanf and functions like these cauz they are somewhere in header files.

Variables declared outside a function have "file scope," meaning they are visible within the file. Variables declared with file scope are visible between their declaration and the end of the compilation unit (.c file) and they implicitly have external linkage and are thus visible to not only the .c file or compilation unit containing their declarations but also to every other compilation unit that is linked to form the complete program.

Global variables can, as the name suggests, be considered to be accessible globally(everywhere)

来源：https://stackoverflow.com/questions/21898770/file-scope-and-global-scope-c-c

标签

c++

scope

global-scope