getline doesnt block the program

[xenoviaquarta]

why is cin.getline doesnt block the program.
a c++ problem again
so i have this 2 snippets:


first:

void messaging()
{
    while(true)
    {
        std::cout << "Enter message: ";
        char message[128];
        std::cin.getline(message,128);
    }
}
int main()
{
unsigned port;
std::string localhost;
std::cout << "Enter port you want to connect to: ";
std::cin >> port;
std::cout << "Enter address or name of host: ";
std::cin >> localhost;

if ( socket.connect(localhost,port) == 0 )
{
std::cout << "Enter your name: ";
char name[128];
std::cin.getline(name,128);
}
sf::Thread thread(&messaging)
thread.launch();

}

so in this first snippet

Enter port you want to connect to: 5000
Enter the address or name of host: 192.168.1.102
Enter your name: Enter message:  // see? it doesnt block the program, it should wait until the user type a name

but when i do this i dont get any problem, the getline blocks the program
works perfectly

void messaging()
{
    while(true)
    {
        std::cout << "Enter message: ";
        char message[128];
        std::cin.getline(message,128);
    }
}
int main()
{
unsigned port = 5000;
std::string localhost = "192.168.1.102";


if ( socket.connect(localhost,port) == 0 )
{
std::cout << "Enter your name: ";
char name[128];
std::cin.getline(name,128);
}
sf::Thread thread(&messaging)
thread.launch();

}

[ShiftLeft]

Why don't you do this instead

    string name;
    getline (cin, name);

[Yanson]

On line 16 you ask the user to enter an ip address. So lets assume the user enters 192.168.1.102. However, the user will also press enter at the end of 192.168.1.102. So the input actually looks like this 192.168.1.102\n. Note the newline at the end.

On line 17 the statement std::cin >> localhost; is executed. The >> operator stops reading when it encounters whitespace. The newline character is considered whitespace. So this statement std::cin >> localhost; reads in 192.168.1.102. This leaves behind a newline character.

On line 23 you call getline std::cin.getline(name,128); which reads until it encounters a newline character, but remember the previous operation left a newline in the input stream, so the first character getline encounters is a newline. So the newline is extracted from the stream but nothing is placed in you name variable.

[xenoviaquarta]

Okay, that was a very good explanation, how about the 2nd snippet? When i put my name and then enter it should skip the first

 enter a message: 

Because newline will left from when i input the name. But that doesnt happen? I just want tp understand. Base from your explanation the output in the 2nd snippet should be

 
Enter your name: myname
Enter message: Enter message:

[Yanson]

The getline function does not leave a newline in the input stream. Only the >> operator does that. The getline function reads up to its delimiter. Witch by default is the newline character. The delimiter is extracted from the stream and discarded, so no newline is left behind.

[xenoviaquarta]

so i cant have >> operator before getline?

[Yanson]

You can put a std::cin.ignore(); statement before a getline statement to remove the newline e.g.

std::cin.ignore();
std::getline(std::cin, port);

[xenoviaquarta]

it works when i put std::cin.ignore(); i tested it when i put

std::cout << std::endl;
std::cout << std::endl;
std::cout << std::endl;
std::cin.ignore();

it flushes all the characters left from the previous even though the default delim of cin.ignore is EOF and the streamsize to ignore is only 1
http://www.cplusplus.com/reference/istream/istream/ignore/


and about this too:

std::cout << "Enter port you want to connect to: ";
std::cin >> port;
std::cout << "Enter address or name of host: ";
std::cin >> localhost;

, it works properly without skipping the

std::cin >> localhost;

ive fixed it all but can you answer my last question? thank you very much

[Yanson]

This is only a problem when you have a statement such as std::cin >> var; followed by a getline statement e.g.

std::cin >> var;
std::getline(std::cin, port);

In this example

std::cout << "Enter port you want to connect to: ";
std::cin >> port;
std::cout << "Enter address or name of host: ";
std::cin >> localhost;

Lets assume the user enters 80 in response to the first std::cout statement. As discussed before a newline is also entered so the input looks like this 80\n.

On line 2 the 80 is extracted leaving a newline behind.

The >> operator however, skips over leading whitespace. So on line 4 the newline left behind on line 2 is simply ignored.