Safe File Pointer

The class constructor and destructor semantics can be used to provide safe files. C++ semantics guarantee that once the constructor for a variable completes (such as for local file on Line [15] below) any control transfer out of the variable's scope will cause a call to its destructor. This is used in the following code to close the file; thus, the file use is safe as the function properly releases acquired resources.

[ 1]   #include "stdio.h"
[ 2]   class SafeFile
[ 3]   {
[ 4]   private:
[ 5]       FILE *f;
[ 6]   
[ 7]   public:
[ 8]       SafeFile(const char *name, const char *mode) 
               {f = fopen(name, mode);}
[ 9]       ~SafeFile() {fclose (f);}
[10]       operator FILE*() {return f;}
[11]   };
[12]   
[13]   f(char *buf, int size)
[14]   {
[15]       SafeFile file("data", "r");
[16]   
[17]       if (fread(buf, 1, size, file) != size)
[18]           return(-1);
[19]       ...
[20]       fclose(file);
[21]   }

• The return on Line [18] forces a call to SafeFile's destructor (Line [9]), which closes the file. If FILE* replaces SafeFile, then the file would remain open and function f would fail to release the file descriptor, whenever Line [18] is executed.

• The operator operatorFILE defined on Line [10] is used to convert an instance of class SafeFile to type FILE*. This allows a variable of class SafeFile, such as file, to be used in any context requiring a FILE*.