String to char array in c++

Question

I was making a simple program in c++ to convert a string to a char array and then print it out. My code is:

string UserImput;
int lenght;
 
 void Test()
{
 getline(cin, UserImput);
 
 lenght = UserImput.size();
 char char_array[lenght + 1];
 copy(UserImput.begin(), UserImput.end(), char_array);
 
 cout << char_array;
 
}

The error I am getting is "expression must have a costant value" and I do not know why.

• cigien: variable length arrays are not allowed in c++. The size of an array must be known at compile time, you aren't allowed to calculate it from run-time input.
• Michael Dorgan: Or you need to use new/delete to create your runtime sized buffer. You will also need to add a NUL terminator to your char array to print it correctly after your copy, unless you print it manually 1 character at a time.
• HolyBlackCat: Why do you want to do this? std::strings already use arrays under the hood. Use .c_str() go get the pointer to its first element (or .data() for a writeable pointer).
• user4581301: Variable Length Arrays are optionally allowed in C programs and by by extension by some C++ compilers, notably g++, so don't be surprised when you see them in examples and tutorials. Just don't expect the example to work under your compiler.
• drescherjm: Related: https://stackoverflow.com/questions/1887097/why-arent-variable-length-arrays-part-of-the-c-standard

Answer 1

In char char_array[lenght + 1];, lenght + 1 is not a compile-time constant. The size of an array must be known at compile-time. Since the value of length is not known until runtime, you will need to allocate memory dynamically in this situation. For example:

char* char_array = new char[lenght + 1];

Don't forget to delete the array when you are done:

delete[] char_array;

• Remy Lebeau: Rather than using new[] directly, you should use std::vector instead, then you don't have to worry about calling new[]/delete[] manually, eg: std::vector<char> char_array(lenght + 1); std::copy(UserImput.begin(), UserImput.end(), char_array.begin()); std::cout << char_array.data();

Answer 2

You can use:

 char* c = _strdup(UserImput.c_str());

But I wonder - why? Just to output it to cout? Then you can do:

cout << UserImput.c_str();

• Remy Lebeau: Or better, cout << UserImput; since operator<< has an overload for std::string
• user4581301: Side note: Remember that the buffer returned by strdup needs to be freeed, not deleteed.