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# std::strstreambuf::underflow

From cppreference.com
< cpp‎ | io‎ | strstreambuf

protected: virtual streambuf<char>* setbuf(char* s, std::streamsize n); |  |

If `s` is a null pointer and `n` is zero, this function has no effect.

Otherwise, the effect is implementation-defined: some implementations do nothing, while some implementations deallocate the dynamic member array used as the buffer and begin using the user-supplied character array of size `n`, whose first element is pointed to by `s`.

This function is protected virtual, it may only be called through `pubsetbuf()` or from member functions of a user-defined class derived from `std::strstreambuf`.

### [edit] Parameters

s | - | pointer to the first byte in the user-provided buffer
n | - | the number of bytes in the user-provided buffer

### [edit] Return value

*this

### [edit] Example

Implementation test to check if setbuf() is supported on a dynamic strstream (output obtained with Sun Studio)

Run this code

    #include <strstream>
    #include <iostream>

    int main()
    {
        char a[100] = {};
        std::strstream str;
        str.rdbuf()->pubsetbuf(a, sizeof a);
        str << "Test string" << std::ends;
        std::cout << "user-provided buffer holds '" << a << "'\n";
    }

Possible output:

    user-provided buffer holds 'Test string'

### [edit] See also

pubsetbuf | invokes setbuf() (public member function of `std::basic_streambuf`) [edit]
setbuf [virtual] | replaces the buffer with user-defined array, if permitted (virtual protected member function of `std::basic_streambuf`) [edit]
setbuf [virtual] | attempts to replace the controlled character sequence with an array (virtual protected member function of `std::basic_stringbuf`) [edit]
setbuf [virtual] | provides user-supplied buffer or turns this filebuf unbuffered (virtual protected member function of `std::basic_filebuf`) [edit]

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