Question

I have this code:

std::string name = "kingfisher";
char node_name[name.size()+1];
strcpy(node_name,name.c_str());
node_name[name.size()] = '\0';

It worked well in DevC++, but in Visual C++, i got a problem named “name.size() must be constant value”! How to solve the problem? I know that i have to use a const value in declaration of node_name, but sometimes (like the case above) i cant! thanks!

Answer 1

char node_name[name.size()+1];

As the value of name.size() is not known at compile time, in the above declaration,node_name is variable length array (VLA) which is not allowed in ISO C++.

In DevC++, it compiles and works, because it provides VLA feature as extension, which is enabled in your compilation configuration.

Use std::string, or char * along with new[]/delete[], whatever suits your need.

In your particular case, i.e if you know the string-literal already, then you could write this:

char node_name[] = "kingfisher"; //this works great!

However, if the string value isn’t known and you want to copy it from somewhere, then do this:

char *node_name = new char[name.size()+1];
std::strncpy(node_name, name.c_str(), name.size()+1); //use strncpy

//work with node_name

//must deallocate the memory
 delete []node_name; //not `delete node_name;`

Use std::strncpy instead of std::strcpy, as the former takes the buffer-size also as third argument, as shown above, and the latter doesn’t (which is unsafe usually; not in this case though).