Trouble Understanding How to Pass a String in C++ - c++

Question

Could somebody possibly breakdown the proper way to pass a string to a function, have that function put it into a vector and another function print contents of the vector? I'm sure there are tons of duplicates and I have looked at them all(mostly) and still have not been able to apply any of it to this problem, or at least it seems that way to me.
#include <cstdio>
#include <string>
#include <vector>
using namespace std;
static vector<string> added_messages;
static void addMessage(string message);
static void displayMessages();
int main()
{
string message = "Testing 1 2 3";
addMessage(message);
//printf("%s\n", message);
return 0;
}
void addMessage(string s)
{
added_messages.push_back(s);
//printf("%s\n", s);
}
void displayMessages()
{
if (added_messages.size() != 0) {
for (string i : added_messages)
printf("%s\n", i);
}
added_messages.clear();
}
It mostly prints out garbage:4÷/4⌠/
I'm fairly new to C++ and coming from Java, I just can't figure this out. Thanks.

Answer 1

Get in the habit of declaring string parameters as const reference.
Instead of this:
void addMessage(string s)
{
...
}
This:
void addMessage(const string& s)
{
...
}
Not only does this avoid making a copy of the string, it also lets you pass string instances as well as string literals and char* variables that point to strings. That is, the above enables all of the following:
addMessage("Foobar");
const char* psz = <some other string>
addMessage(psz);
std::string s = "A string";
addMessage(s);
And then to print the string correctly with printf, use the .c_str() member function to get the pointer address of the contents.
Instead of this:
printf("%s\n", s);
This:
printf("%s\n", s.c_str());
And it goes without saying that cout is preferred over printf:
cout << s << endl;
Putting it altogether:
static vector<string> added_messages;
static void addMessage(const string& message);
static void displayMessages();
int main()
{
string message = "Testing 1 2 3";
const char* psz = "Yet, another messsage);
addMessage(message);
addMessage("Another Message as a string literal);
addMessage(psz);
// sample printf statements
printf("%s\n", message.c_str());
printf("%s\n", psz);
// sample cout statements - notice it can handle both pointers and string instances
cout << message << endl;
cout << psz << endl;
cout << "Hello World" << endl;
return 0;
}
void addMessage(const string& s)
{
added_messages.push_back(s);
printf("%s\n", s.c_str());
}
void displayMessages()
{
for (const string& i : added_messages)
{
printf("%s\n", i.c_str());
}
added_messages.clear();
}

Answer 2

You can't use printf("%s") on string() object - this is the reason you are having the garbage in the output. "%s" expects C string, ie a pointer to char ended with a zero byte.
If you need to print a string() object, use c_str() method to get a C string representation, ie in your case i.c_str(). C++ is mixing C concepts with C++ concepts, so be aware. Use iostream header and std::cout << i to output your object is a C++ way.
About argument passing. In your example, you are doing it okay, but keep in mind that in your case you are doing it by value, ie a copy of the object is created. More efficient is passing by reference.

Answer 3

This should work:
for(std::size_t i = 0; i < added_messages.size(); i++)
printf("%s\n",added_messages[i].c_str());