std::basic_string<CharT,Traits,Allocator>::append

basic_string& append( size_type count, CharT ch );	(1)	(constexpr since C++20)
basic_string& append( const CharT* s, size_type count );	(2)	(constexpr since C++20)
basic_string& append( const CharT* s );	(3)	(constexpr since C++20)
template< class SV > basic_string& append( const SV& t );	(4)	(since C++17) (constexpr since C++20)
template< class SV > basic_string& append( const SV& t, size_type pos, size_type count = npos );	(5)	(since C++17) (constexpr since C++20)
basic_string& append( const basic_string& str );	(6)	(constexpr since C++20)
basic_string& append( const basic_string& str, size_type pos, size_type count );	(7)	(until C++14)
basic_string& append( const basic_string& str, size_type pos, size_type count = npos );	(7)		(since C++14) (constexpr since C++20)
template< class InputIt > basic_string& append( InputIt first, InputIt last );	(8)	(constexpr since C++20)
basic_string& append( std::initializer_list<CharT> ilist );	(9)	(since C++11) (constexpr since C++20)

Appends additional characters to the string.

1) Appends count copies of character ch.

2) Appends characters in the range [s, s + count).

If [s, s + count) is not a valid range, the behavior is undefined.

3) Equivalent to return append(s, Traits::length(s));.

4,5) Appends characters in a string view sv constructed from t.

If only t is provided, all characters in sv are appended.
If pos is also provided:
- If count is npos, all characters in sv starting from pos are appended.
- Otherwise, the std::min(count, sv.size() - pos) characters in sv starting from pos are appended.

These overloads participate in overload resolution only if all following conditions are satisfied:

std::is_convertible_v<const SV&, std::basic_string_view<CharT, Traits>> is true.
std::is_convertible_v<const SV&, const CharT*> is false.

4) Equivalent to std::basic_string_view<CharT, Traits> sv = t;return append(sv.data(), sv.size());.

5) Equivalent to std::basic_string_view<CharT, Traits> sv = t;return append(sv.substr(pos, count));.

6,7) Appends characters in another string str.

If only str is provided, all characters in it are appended.
If pos is also provided:
- If count is npos, all characters in str starting from pos are appended.
- Otherwise, the std::min(count, str.size() - pos) characters in str starting from pos are appended.

6) Equivalent to return append(str.data(), str.size());.

7) Equivalent to return append(std::basic_string_view<CharT, Traits>(str).substr(pos, count));. (since C++20)

8) Equivalent to return append(basic_string(first, last, get_allocator()));.

This overload has the same effect as overload (1) if `InputIt` is an integral type.	(until C++11)
This overload participates in overload resolution only if `InputIt` satisfies the requirements of LegacyInputIterator.	(since C++11)

9) Equivalent to return append(ilist.begin(), ilist.size());.

Parameters

count	-	number of characters to append
ch	-	character value to append
s	-	pointer to the character string to append
t	-	object convertible to `std::basic_string_view` with the characters to append
pos	-	the index of the first character to append
str	-	string to append
first, last	-	range of characters to append
ilist	-	initializer list with the characters to append

Return value

*this

Complexity

There are no standard complexity guarantees, typical implementations behave similar to std::vector::insert().

Exceptions

If the operation would cause size() to exceed max_size(), throws std::length_error.

5) If pos > sv.size() is true, throws std::out_of_range.

7) If pos > str.size() is true, throws std::out_of_range.

If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee).

Example

#include <cassert>
#include <string>

int main()
{
    std::string str = "std::string";
    const char* cptr = "C-string";
    const char carr[] = "range";

    std::string result;

    // 1) Append a char 3 times.
    // Note: This is the only overload accepting “CharT”s.
    result.append(3, '*');
    assert(result == "***");

    // 2) Append a fixed-length C-string
    result.append(cptr, 5);
    assert(result == "***C-str");

    // 3) Append a null-terminated C-string
    // Note: Because “append” returns *this, we can chain calls together.
    result.append(1, ' ').append(cptr);
    assert(result == "***C-str C-string");

    // 6) Append a whole string
    result.append(1, ' ').append(str);
    assert(result == "***C-str C-string std::string");

    // 7) Append part of a string
    result.append(str, 3, 2);
    assert(result == "***C-str C-string std::string::");

    // 8) Append range
    result.append(&carr[2], &carr[3]);
    assert(result == "***C-str C-string std::string::n");

    // 9) Append initializer list
    result.append({'p', 'o', 's'});
    assert(result == "***C-str C-string std::string::npos");
}

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 847	C++98	there was no exception safety guarantee	added strong exception safety guarantee
LWG 2250	C++98	the behavior of overload (7) was undefined if `pos > str.size()` is `true`	always throws an exception in this case
LWG 2788	C++98	overload (8) used a default constructed allocator to construct the temporary string	obtains the allocator from `get_allocator()`
LWG 2946	C++17	overload (4) causes ambiguity in some cases	avoided by making it a template

append_range (C++23)	appends a range of characters to the end (public member function)
operator+=	appends characters to the end (public member function)
strcat	concatenates two strings (function)
strncat	concatenates a certain amount of characters of two strings (function)
wcscat	appends a copy of one wide string to another (function)
wcsncat	appends a certain amount of wide characters from one wide string to another (function)