C++ forbids variable-size array

Question

I am using some existing code that someone else has written, and I cannot get it to compile (limited C experience here but I am trying to learn!).

utilities.cc

#include "utilities.h"
FILE *open_file(char *filename, const char*extension, const char *access)
{
  char string[MAX_STR_LEN];
  FILE *strm = NULL;

  if(filename[0]=='\0')
   {
      printf("\n INPUT FILENAME (%s) > ",access);
      fgets(string,MAX_STR_LEN,stdin);
      sscanf(string,"%s",filename);
      printf(" FILE %s opened \n", filename);
   }
   int len=strlen(filename);

   if( len + strlen(extension) >= MAX_STR_LEN)
   {
      printf("\n ERROR: String Length  of %s.%s Exceeds Maximum",
              filename, extension);
      return(NULL);
   } 

   // char *filename1 = new(char[len+strlen(extension)+1]);

   const int filenameLength = len+strlen(extension)+1;
   char *filename1 = new(char[filenameLength]);

   strcpy(filename1,filename); // temp filename for appending extension

   /* check if file name has .extension    */
   /* if it does not, add .extension to it */
   int i=len-1;
   while(i > 0 && filename[i--] != '.');
   //   printf("\n Comparing %s to %s", extension, filename+i+1);
   if(strcmp(extension, filename+i+1)  )
      strcat(filename1,extension);
   if( (strm = fopen(filename1, access) ) == NULL )
   {
      printf("\n ERROR OPENING FILE %s (mode %s)", filename1,access);
   }
   delete(filename1);
   return(strm);
}

Here is the error.

Compiling utilities.cc ...
src/utilities.cc: In function ‘FILE* open_file(char*, const char*, const char*)’:
src/utilities.cc:251: error: ISO C++ forbids variable-size array
gmake: *** [/home/landon/geant4/work/tmp/Linux-g++/exampleN01/utilities.o] Error 1

The error on line 251 refers to

char *filename1 = new(char[filenameLength]);

If you need any additional information let me know please.

Are you passing -Werror which treats warnings as errors? g++ allows VLAs as an extension. — Jesse Good
– Jesse Good, Commented Jul 7, 2012 at 23:22
Really, this is C, not C++. You're just using a C++ compiler. — Ed Swangren
– Ed Swangren, Commented Jul 7, 2012 at 23:22
@Caleth: See en.cppreference.com/w/cpp/keyword/new and en.cppreference.com/w/cpp/keyword/delete. They've been keywords since the earliest days of C++. — MSalters
– MSalters, Commented Mar 11 at 12:26
@MSalters assuming this is a C program, not a C++ program, then utilities.h could define macros/functions for new and delete. Note the parentheses — Caleth
– Caleth, Commented Mar 11 at 13:02

mfontanini · Accepted Answer · 2012-07-07 23:22:01Z

33

The error is correct. VLA(variable size arrays) are forbidden in C++. This is a VLA:

char filename1char[filenameLength];

What you probably meant is this:

char *filename1 = new char[filenameLength];

Which is not a VLA, but an array of chars allocated on the heap. Note that you should delete this pointer using operator delete[]:

delete[] filename1;

answered Jul 7, 2012 at 23:22

mfontanini

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2 Comments

user1509364 Over a year ago

I should replace delete(filename1) with delete[] filename1?

Qaz Over a year ago

@user1509364, delete what you new, delete[] what you new[].

mathematician1975 · Accepted Answer · 2012-07-07 23:43:44Z

17

Try this instead

    char *filename1 = new char[filenameLength];

you cannot create an array as a local variable length array on the stack like this

    char filename1[filenamelength];

unless filenamelength is declared as const.

Also as you have allocated memory for an array, you should free the memory using

   delete [] filename1;

otherwise you will have a memory leak. Also it is not essential to have parentheses around your return values;

edited Jul 7, 2012 at 23:43

answered Jul 7, 2012 at 23:21

mathematician1975

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Comments

user1095108 · Accepted Answer · 2025-03-12 06:27:03Z

5

They are forbidden, but a workaround is to use a stack allocator, for example:

http://howardhinnant.github.io/stack_alloc.html

You can use the stack allocator with a std::vector (or with some other container, or just directly) and you've got yourself a VLA.

EDIT: The a look at a ring buffer class for an alternative container, but that approach is more involved.

edited Mar 12 at 6:27

answered Jan 17, 2014 at 11:29

user1095108

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