Iso c++ forbids variable length array - simple way to solve it

Question

For the problem “iso c++ forbids variable length array.” I tried to fix it, but It doesn't work and returns the result I want. Here is my program:

#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("
 INPUT FILENAME (%s) > ",access);
  fgets(string,MAX_STR_LEN,stdin);
  sscanf(string,"%s",filename);
  printf(" FILE %s opened 
", filename);
  }
  int len=strlen(filename);
 
  if( len + strlen(extension) >= MAX_STR_LEN)
  {
  printf("
 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("
 Comparing %s to %s", extension, filename+i+1);
  if(strcmp(extension, filename+i+1) )
  strcat(filename1,extension);
  if( (strm = fopen(filename1, access) ) == NULL )
  {
  printf("
 ERROR OPENING FILE %s (mode %s)", filename1,access);
  }
  delete(filename1);
  return(strm);
 }

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

and

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

has occurred. I've checked the entire command line but still can't find the mistake.

dttutoria · Accepted Answer

The cause: The error: Iso c++ forbids variable length array" happens because VLA (variable size arrays) are not allowed in C++. You can't make an array like this on the stack as a local variable length array:

char filename1[filenamelength];

Solution: Instead, try this:

char *filename1 = new char[filenameLength];

In addition, when you've already given memory for an array, you could perhaps clear memory by using

delete [] filename1;

Mia Teixeira · Answer

This error is correct. C++ prohibits variable size arrays (VLA). This is a VLA.

char filename1char[filenameLength];

Most likely, you meant this:

char *filename1 = new char[filenameLength];

This is not a VLA but an array chars that are allocated to the heap. This pointer should be deleted using operator delete[]

delete[] filename1;

Chloë Francois · Answer

These are prohibited, but you can use a stack allocator to get around it.

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

The stack allocator can be used with a ::std::vector or with another container, or directly. You'll have a VLA.

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Iso c++ forbids variable length array – simple way to solve it