c++ append one character from a string to another string

Question

#include <string>
#include <iostream>

int main()
{
    std::string test("hello world ");
    std::string label("label test");
    test.append(&label[3]);    //----- #1
    std::cout << test << std::endl;
}

for the above code, what I expected is "hello world e", but in fact, its output is "hello world el test". It append all the characters after position 3 to my string test.

At the position #1, if I don't put the & sign, there will be compilation error:

str_append.cpp:10:10: error: no matching member function for call to 'append'
    test.append(label[3]);
    ~~~~~^~~~~~
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/string:1516:19: note: candidate function not viable: no known conversion
      from 'value_type' (aka 'char') to 'const value_type *' (aka 'const char *') for 1st argument; take the address of the argument with &
    basic_string& append(const value_type* __s);
                  ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/string:1513:19: note: candidate function not viable: no known conversion
      from 'value_type' (aka 'char') to 'const std::__1::basic_string<char>' for 1st argument
    basic_string& append(const basic_string& __str);
                  ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/string:1525:9: note: candidate function template not viable: requires 2
      arguments, but 1 was provided
        append(_InputIterator __first, _InputIterator __last);
        ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/string:1532:9: note: candidate function template not viable: requires 2
      arguments, but 1 was provided
        append(_ForwardIterator __first, _ForwardIterator __last);
        ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/string:1515:19: note: candidate function not viable: requires 2
      arguments, but 1 was provided
    basic_string& append(const value_type* __s, size_type __n);
                  ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/string:1517:19: note: candidate function not viable: requires 2
      arguments, but 1 was provided
    basic_string& append(size_type __n, value_type __c);
                  ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/string:1514:19: note: candidate function not viable: requires at least 2
      arguments, but 1 was provided
    basic_string& append(const basic_string& __str, size_type __pos, size_type __n=npos);
                  ^
1 error generated.

The only way i can solve this is to change the #1 line to :

test += label[3];

So, I want to know what the logic behind this? Can't label[3] return a single character? why it fails when i just use test.append(label[3]);. And why test += label[3]; succeeds?

Thanks in advance.

Yet another "I didn't read the documentation" question. What is wrong with the world? downvotes and cries — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Oct 18, 2015 at 14:38

Lightness Races in Orbit · Accepted Answer · 2015-10-18 14:35:23Z

You aren't appending a character; you're appending a C-string. That C-string is provided by the pointer &label[3], which points to the following data: {'e','l',' ','t','e','s','t','\0'}.

To append a character you'd just pass label[3] itself. However, to do this you need to use basic_string& append(size_type count, CharT ch), or its convenient equivalent (when count==1), void push_back(CharT ch).

The operator+= is functionally analogous to push_back, which is why it worked when you tried it.

So:

#include <string>
#include <iostream>

int main()
{
    std::string test("hello world ");
    std::string label("label test");

    test.append(1, label[3]);  // or...
    test.push_back(label[3]);  // or...
    test += label[3];

    std::cout << test << std::endl;
}

In brief, read the documentation to find out what members std::string has, and how to use them.

Mathias Rav · Accepted Answer · 2015-10-18 14:24:59Z

3

Because there is no std::basic_string::append() overload that takes a single character. See for yourself here.

Basically, std::basic_string::append() is not designed to add a single character to the back of a string, but characters. The std::basic_string::push_back() is designed for this purpose instead. Or you may just use operator + which is applicable to both a single character and characters.

On cppreference, the descriptions for each function are (emphasis mine):

append(): Appends characters to the end
push_back(): Appends a character to the end
operator +: Concatenates two strings or a string and a char

edited Oct 18, 2015 at 14:24

Mathias Rav

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answered Oct 18, 2015 at 14:11

Lingxi

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Comments

MikeCAT · Accepted Answer · 2015-10-18 14:11:31Z

0

label[3] will return a single character (structly speaking, a reference of it), but unfortunately std::string doesn't have a function append that takes one character.

test.append('a'); also fails.

std::string has operator+= that takes one character, so test += label[3]; succeeds.

answered Oct 18, 2015 at 14:11

MikeCAT

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2 Comments

Lingxi Over a year ago

Actually, it is operator +. std::basic_string does not have a operator += overload.

MikeCAT Over a year ago

@Lingxi N3337 says that basic_string has basic_string& operator+=(charT c); (21.4.6.1 basic_string::operator+=)

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