A couple of questions

I am learning the STL and have some questions.

Code:

#include <iostream>
#include <vector>
#include <string>
using namespace std;

int main()
{
	vector<string> vec;
	vec.push_back("test");

	string s(vec[0]);	//turn vec[0] into a string named s
	std::string c( s.size(), char() );
	cout << c << endl;
	copy( s.begin(), s.end(), c.begin() );
	cout << c << endl;

	
	return 0;
}

1. In the first cout, why doesn't c print out?
2. If I eliminate std::string and define string c above, this doesn't work. I get an error: no match for call to ‘(std::string) (size_t, char)’. What is it that std::string is doing in this case?

Because at first c contains only characters '\0' which are non-printable.

Well, for me it outputs spaces, which you could see when you printed something around it:

Code:

cout << "'" << c << "'" << endl;

Because at first c contains only characters '\0' which are non-printable.

So is this similar to how C puts '\0' at the end of an array only at the beginning of a string? And why do I need std::string in front of c( s.size(), char() );? If I declare c as a string and remove std::string, I get an error.

Originally Posted by swappo

And why do I need std::string in front of c( s.size(), char() );? If I declare c as a string and remove std::string, I get an error.

I am not sure what you mean. Please post the smallest and simplest program that demonstrates that error.

Here is the code with the changes as compared to the original in the first post.

Code:

#include <iostream>
#include <vector>
#include <string>
using namespace std;

int main()
{
	vector<string> vec;
	vec.push_back("test");
	string c;//added

	string s(vec[0]);	
	c( s.size(), char() );//deleted std::string
	cout << c << endl;
	copy( s.begin(), s.end(), c.begin() );
	cout << c << endl;

  	
	return 0;
}

Code:

string_copy1.cpp: In function ‘int main()’:
string_copy1.cpp:13: error: no match for call to ‘(std::string) (size_t, char)’

Also, If I change some of the code to the following:

Code:

std::string c( s.size(), char() );
	for(int i = 0; i < c.size(); i++)
		cout << c[i];
	cout << endl << c.size();

I find that, cout << c[i];, produces all spaces yet c.size() prints out the number 4. Is c( s.size(), char() ) an integer? I hope I've explained this well enough. I am really confused on this.

You can only call the constructor when you define the variable. Just like you had

Code:

std::string c( s.size(), char() );

you can have

Code:

string c( s.size(), char() );

The former calls the constructor and the latter calls operator () (of which there is no defined for std::string).