C++ strcpy non-constant expression as array bound

Question

I turned back to C++ after a long time in C#, PHP and other stuff and I found something strange:

temp.name = new char[strlen(name) + strlen(r.name) + 1];

this compiles

temp.name = (char *)malloc(sizeof(char[strlen(name) 
     + strlen(r.name) + 1]));

this doesn't (temp.name is a char *)

The compiler error is

error C2540: non-constant expression as array bound

Does anyone know what the problem might be and how it might be remedied? Thank you.

Answer 1

sizeof(...) expects a constant compile-time expression. strlen is not a compile-time expression, it is a function which needs to be executed to get a result. Therefore, the compiler is not able to reserve sufficient storage for an array declared like this:

char c[strlen("Hello")];

Although the length of the string is clearly 5, the compiler does not know.

To avoid this pitfall, do not use sizeof here. Instead:

char* c = (char*)malloc(strlen(name)+strlen(rname)+1);

This gives you a pointer to n bytes in return. sizeof(char)==1 is always true, so the number of bytes in the buffer equals the number of chars you can store in it. To malloc arrays of a different type, multiply with the static size of one array element:

int* c = (int*) malloc(sizeof(int)*100);

This is Ok, because sizeof is applied to a compile-time expression. Of course, the C++ way is much cleaner:

int* c = new int[100];

Answer 2

The problem is char[...] which is an array type and in C++ (and C89) array sizes need to be compile-time constants. You should probably use std::string instead of allocating the memory manually by new[] or malloc() , but if you prefer to use manual allocation, calculate the size directly as the number of characters instead of using arrays and sizeof to do it.

Answer 3

malloc needs a size_t as its input, meaning you need to calculate the actual size and pass that in instead of specifying the type:

temp.name = (char *) malloc( (strlen(name) + strlen(r.name) + 1)) * sizeof(char));

You should probably be using new anyway, so I don't see any real issues.