C++ forbids variable-size array

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.

3 Answers

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; 

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;

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

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.