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How to append a char to a std::string?


Question

The following fails with the error prog.cpp:5:13: error: invalid conversion from ‘char’ to ‘const char*’

int main()
{
  char d = 'd';
  std::string y("Hello worl");
  y.append(d); // Line 5 - this fails
  std::cout << y;
  return 0;
}

I also tried, the following, which compiles but behaves randomly at runtime:

int main()
{
  char d[1] = { 'd' };
  std::string y("Hello worl");
  y.append(d);
  std::cout << y;
  return 0;
}

Sorry for this dumb question, but I've searched around google, what I could see are just "char array to char ptr", "char ptr to char array", etc.

2016/08/09
1
177
8/9/2016 8:20:19 PM

Accepted Answer

y += d;

I would use += operator instead of named functions.

2009/09/24
228
9/24/2009 2:29:28 PM

Use push_back():

std::string y("Hello worl");
y.push_back('d')
std::cout << y;
2009/09/24

To add a char to a std::string var using the append method, you need to use this overload:

std::string::append(size_type _Count, char _Ch)

Edit : Your're right I misunderstood the size_type parameter, displayed in the context help. This is the number of chars to add. So the correct call is

s.append(1, d);

not

s.append(sizeof(char), d);

Or the simpliest way :

s += d;
2009/09/24

In addition to the others mentioned, one of the string constructors take a char and the number of repetitions for that char. So you can use that to append a single char.

std::string s = "hell";
s += std::string(1, 'o');
2009/09/24

I test the several propositions by running them into a large loop. I used microsoft visual studio 2015 as compiler and my processor is an i7, 8Hz, 2GHz.

    long start = clock();
    int a = 0;
    //100000000
    std::string ret;
    for (int i = 0; i < 60000000; i++)
    {
        ret.append(1, ' ');
        //ret += ' ';
        //ret.push_back(' ');
        //ret.insert(ret.end(), 1, ' ');
        //ret.resize(ret.size() + 1, ' ');
    }
    long stop = clock();
    long test = stop - start;
    return 0;

According to this test, results are :

     operation             time(ms)            note
------------------------------------------------------------------------
append                     66015
+=                         67328      1.02 time slower than 'append'
resize                     83867      1.27 time slower than 'append'
push_back & insert         90000      more than 1.36 time slower than 'append'

Conclusion

+= seems more understandable, but if you mind about speed, use append

2017/06/28

Try the += operator link text, append() method link text, or push_back() method link text

The links in this post also contain examples of how to use the respective APIs.

2009/09/24

Source: https://stackoverflow.com/questions/1472048
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