在C ++中使用cin.get（）时，如何不限制用户可以输入的字符数？

Question

Im trying to get a users input using cin.get() but I dont want to limit the amount of characters that they can enter. How can I do this?

EDIT: I guess a better way to phrase this would be: How can I dynamicaly change the character array to fit the length of the users input?

Answer 1

This is a strange requirement for a C++ program. You can of course go the C way and simply keep on getting more memory whenever your input outgrows the currently available memory. It goes something like this ( warning: code fragments ahead ):

while(cin.get(c)) {
    if (cur_pos == cur_len) {
        cur_len = grow_charbuf(buffer, cur_len);
    }
    buffer[cur_pos++] = c;
}   

Here, the grow function is where it gets ugly. It needs to allocate a larger piece of memory, copy the contents of the current buffer to the beginning of that, dealocate the memory occupied by the current buffer, and return the new size. For example, something along these lines:

char* new_charbuf(size_t len) {
    return new char [len];
}

size_t grow_charbuf(char* buf, size_t cur_len) {
    size_t new_len = cur_len * 2;
    char* new_buf = new char [new_len];
    // copy old buffer contents to new buffer
    delete[] buf;
    buf = new_buf;
    return new_len;
}

And you can then use it as follows:

cur_len = 1000; // or whatever
char* buffer = new_charbur(cur_len);
// write into the buffer, calling grow_charbuf() when necessary
// and don't forget to free the memory once you are done...
// or don't free it, if the program eventually exits anyway

This is terrible code. It might work, but you should never ever do this in C++ if you can avoid it. Apart from this, I have avoided handling any error conditions or exceptions that this code might cause. It is meant just to illustrate the idea.

Managing your memory manually is a bad idea because it requires a lot of code and is not easy to get right. You can get away with less if your program has a known, limited life-span.

Answer 2

Don't use characters array at all. Use std::string or other standard containers. And of cause learn to use streams.

Here an example. It reads as many characters as the user inputs until the user presses enter. As you cna see, there is no explicite buffer-size required:

/////TEST PUT ANYWHERE IN GLOBAL SCOPE
#include <string>
#include <sstream>
#include <iostream>

using namespace std;

int test()
{
    //SET BP HERE AND STEP THROUGH
    string line;        
    getline(cin,line);
    std::stringstream user_input( line );

    while(!user_input.eof())
    {
        string word;
        user_input >> word;
            cout << word << endl;
    }

    return 0;
}

static int _test = test();
/////END TEST

Answer 3

You need a cin.getline(). In other words you need to have a specified size of char array and use it like so:

Using cin.get()

char str[100];
char sayHello[100];

cin.get(str, 100);
// make sure to add cin.ignore() or program will terminate right before next cin().
cin.ignore(); 

cout << str << endl;
cin.get(sayHello, 100);

cout << sayHello;

or for cin.getline()

char input[100];
cin.ignore(); // stops the sentence from truncating.
cin.getline(input,sizeof(input));

You could also use getline() for strings like so:

string name;
getline(cin, name);

The problem is that in c++ when receiving input your cin looks for the 0 aka the space in your sentence. It then ends thinking that was the end.