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std::strstream::strstream

strstream();
 (1) 
strstream( char* s, int n, std::ios_base::openmode mode =
                           std::ios_base::in | std::ios_base::out );
 (2)

Constructs new input/output strstream and its underlying std::strstreambuf.

1) Default-constructs the underlying std::strstreambuf, which creates a dynamically growing buffer, and initializes the base class with the address of the strstreambuf member.
2) Initialized the base class with the address of the underlying std::strstreambuf member, which is initialized in one of the two possible ways, both of which use a user-provided fixed-size array:
a) if (mode & app) == 0 (the app bit is not set in mode), constructs the buffer by calling strstreambuf(s, n, s). The behavior is undefined if there are less than n elements in the array whose first element is pointed to by s.
b) if (mode & app) != 0 (the app bit is set in mode), constructs the buffer by calling strstreambuf(s, n, s + std::strlen(s)). The behavior is undefined if there are less than n elements in the array whose first element is pointed to by s or if the array does not contain a valid null-terminated character sequence.

Parameters

s - char array to use as the output buffer
n - size of the array to be used for output
mode - specifies stream open mode. It is a bitmask type, the following constants are defined (although only app is used):
Constant Explanation
app seek to the end of stream before each write
binary open in binary mode
in open for reading
out open for writing
trunc discard the contents of the stream when opening
ate seek to the end of stream immediately after open
noreplace (C++23) open in exclusive mode

Example

#include <iostream>
#include <strstream>
#include <string>
 
int main()
{
    // dynamic buffer
    std::strstream s1;
    s1 << 1 << ' ' << 3.14 << " example" << std::ends;
    std::cout << "Buffer holds: '" << s1.str() << "'\n";
    s1.freeze(false);
 
    int n;
    double d;
    std::string w;
    s1 >> n >> d >> w;
    std::cout << "Read back: n = " << n
              << ", d = " << d
              << ", w = '" << w << "'\n";
 
    // static buffer
    char arr[20] = "-1 -3.14 ";
    std::strstream s2(arr, sizeof arr, std::ios_base::app);
    s2 << "another" << std::ends;
    std::cout << "Buffer holds: '" << s2.str() << "'\n";
    s2 >> n >> d >> w;
    std::cout << "Read back: n = " << n
              << ", d = " << d
              << ", w = '" << w << "'\n";
}

Output:

Buffer holds: '1 3.14 example'
Read back: n = 1, d = 3.14, w = 'example'
Buffer holds: '-1 -3.14 another'
Read back: n = -1, d = -3.14, w = 'another'

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 115 C++98 overload (2) only considered the case
mode & app == 0 (== has higher precedence than &) 		considers cases (mode & app) == 0
and (mode & app) != 0

See also

constructs a strstreambuf object
(public member function of std::strstreambuf)
constructs an istrstream object, optionally allocating the buffer
(public member function of std::istrstream)
constructs an ostrstream object, optionally allocating the buffer
(public member function of std::ostrstream)

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