# std::ostrstream::ostrstream

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< cpp‎ | io‎ | ostrstream

ostrstream(); | (1) | (deprecated in C++98) (removed in C++26)
ostrstream( char* s, int n, std::ios_base::openmode mode = std::ios_base::out ); | (2) | (deprecated in C++98) (removed in C++26)

Constructs new 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 write to user-provided fixed-size array:
a) if 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 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 as the output buffer

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

Run this code

    #include <iostream>
    #include <strstream>

    int main()
    {
        std::ostrstream s1; // dynamic buffer
        s1 << 1 << ' ' << 3.14 << " example\n" << std::ends;
        std::cout << s1.str();
        s1.freeze(false);

        char arr[15] = "Hello";

        std::ostrstream s2(arr, sizeof arr, std::ios_base::app);
        s2 << ", world!" << std::ends;
        std::cout << s2.str() << '\n';
        std::cout << arr << '\n'; // streams use the provided arrays
    }

Output:

    1 3.14 example
    Hello, world!
    Hello, world!

### See also

(constructor) | constructs a `strstreambuf` object (public member function of `std::strstreambuf`)
(constructor) | constructs an `istrstream` object, optionally allocating the buffer (public member function of `std::istrstream`)
(constructor) | constructs a `strstream` object, optionally allocating the buffer (public member function of `std::strstream`)

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