Declarations/definitions as statements in C and C++

Question

I was confused when this wouldn\'t compile in C:

int main()
{
    for (int i = 0; i < 4; ++i)
        int a = 5; // A dependent statement may not be declarati         


        

Answer 1

In C++ declarations are statements while in C declarations are not statements. So according to the C grammar in this for loop

for (int i = 0; i < 4; ++i)
    int a = 5;

int a = 5; must be a substatement of the loop. However it is a declaration.

You could make the code to be compiled in C by using the compound statement as for example

for (int i = 0; i < 4; ++i)
{
    int a = 5;
}

though the compiler can issue a diagnostic message saying that the variable a is not used.

One more consequence that in C declarations are not statements. You may not place a label before a declaration in C. For example this program

#include 

int main(void) 
{
    int n = 2;

    L1:
    int x = n;

    printf( "x == %d\n", x );

    if ( --n ) goto L1; 

    return 0;
}

does not compile in C though it compiles as a C++ program. However if to place a null-statement after the label then the program does compile.

#include 

int main(void) 
{
    int n = 2;

    L1:;
    int x = n;

    printf( "x == %d\n", x );

    if ( --n ) goto L1; 

    return 0;
}