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std::setbase

Defined in header <iomanip>
/*unspecified*/ setbase( int base );

Sets the numeric base of the stream. When used in an expression out << setbase(base) or in >> setbase(base), changes the basefield flag of the stream out or in, depending on the value of base:

Values of base other than 8, 10, or 16 reset basefield to zero, which corresponds to decimal output and prefix-dependent input.

Parameters

base - new value for basefield

Return value

An object of unspecified type such that.

  • if out is an object of type std::basic_ostream<CharT, Traits>, the expression out << setbase(base)
    • has type std::basic_ostream<CharT, Traits>&
    • has value out
    • behaves as if it called f(out, base)
  • if in is an object of type std::basic_istream<CharT, Traits>, the expression in >> setbase(base)
    • has type std::basic_istream<CharT, Traits>&
    • has value in
    • behaves as if it called f(in, base)

where the function f is defined as:

void f(std::ios_base& str, int base)
{
    // set basefield
    str.setf(base == 8 ? std::ios_base::oct :
        base == 10 ? std::ios_base::dec :
        base == 16 ? std::ios_base::hex :
        std::ios_base::fmtflags(0), std::ios_base::basefield);
}

Example

#include <iostream>
#include <sstream>
#include <iomanip>
 
int main()
{
    std::cout << "Parsing string \"10 0x10 010\"\n";
 
    int n1, n2, n3;
    std::istringstream s("10 0x10 010");
 
    s >> std::setbase(16) >> n1 >> n2 >> n3;
    std::cout << "hexadecimal parse: " << n1 << ' ' << n2 << ' ' << n3 << '\n';
 
    s.clear();
    s.seekg(0);
 
    s >> std::setbase(0) >> n1 >> n2 >> n3;
    std::cout << "prefix-dependent parse: " << n1 << ' ' << n2 << ' ' << n3 << '\n';
 
    std::cout << "hex output: " << std::setbase(16)
              << std::showbase << n1 << ' ' << n2 << ' ' << n3 << '\n';
}

Output:

Parsing string "10 0x10 010"
hexadecimal parse: 16 16 16
prefix-dependent parse: 10 16 8
hex output: 0xa 0x10 0x8

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 183 C++98 setbase could only be used with streams
of type std::ostream or std::istream
usable with any
character stream

See also

changes the base used for integer I/O
(function)
controls whether prefix is used to indicate numeric base
(function)

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https://en.cppreference.com/w/cpp/io/manip/setbase