std::ios_base::width

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< cpp‎ | io‎ | ios base
 
C++
 
Input/output library
 
std::ios_base
 
streamsize width() const;
 (1)
streamsize width( streamsize new_width );
 (2)

Manages the minimum number of characters to generate on certain output operations and the maximum number of characters to generate on certain input operations.

1) Returns the current field width.
2) Sets the field width to the given one. Returns the previous field width.

Parameters

new_width - new field width setting

Return value

The field width before the call to the function.

Notes

Some I/O functions call width(0) before returning, see std::setw (this results in this field having effect on the next I/O function only, and not on any subsequent I/O).

The exact effects this modifier has on the input and output vary between the individual I/O functions and are described at each operator<< and operator>> overload page individually.

Example

Run this code
#include <algorithm>
#include <array>
#include <iomanip>
#include <iostream>
#include <span>
#include <string_view>
using namespace std::string_view_literals;
 
constexpr std::array table_header =
{
    "Language"sv, "Author"sv, "Birthdate"sv, "RIP date"sv
};
 
using row_t = std::array<std::string_view, table_header.size()>;
using widths_t = std::array<std::size_t, table_header.size()>;
 
constexpr std::array table_body = std::to_array<row_t>
({
    {"C", "Dennis Ritchie", "1941-09-09", "2011-10-12"},
    {"C++", "Bjarne Stroustrup", "1950-12-30"},
    {"C#", "Anders Hejlsberg", "1960-12-02"},
    {"Python", "Guido van Rossum", "1956-01-31"},
    {"Javascript", "Brendan Eich", "1961-07-04"}
});
 
constexpr widths_t calculate_column_widths(std::span<const row_t> table)
{
    widths_t widths{};
    for (const row_t& row : table)
        for (size_t i = 0; i != row.size(); ++i)
            widths[i] = std::max(widths[i], row[i].size());
    return widths;
}
 
void print_row(const row_t& row, const widths_t& widths)
{
    std::cout << '|';
    for (size_t i = 0; i != row.size(); ++i)
    {
        std::cout << ' ';
        std::cout.width(widths[i]);
        std::cout << row[i] << " |";
    }
    std::cout << '\n';
};
 
void print_break(const widths_t& widths)
{
    const std::size_t margin = 1;
    std::cout.put('+').fill('-');
    for (std::size_t w : widths)
    {
        std::cout.width(w + margin * 2);
        std::cout << '-' << '+';
    }
    std::cout.put('\n').fill(' ');
};
 
int main()
{
    constexpr widths_t widths = calculate_column_widths(table_body);
 
    std::cout.setf(std::ios::left, std::ios::adjustfield);
    print_break(widths);
    print_row(table_header, widths);
    print_break(widths);
    for (const row_t& row : table_body)
        print_row(row, widths);
    print_break(widths);
}

Output: 

+------------+-------------------+------------+------------+
| Language   | Author            | Birthdate  | RIP date   |
+------------+-------------------+------------+------------+
| C          | Dennis Ritchie    | 1941-09-09 | 2011-10-12 |
| C++        | Bjarne Stroustrup | 1950-12-30 |            |
| C#         | Anders Hejlsberg  | 1960-12-02 |            |
| Python     | Guido van Rossum  | 1956-01-31 |            |
| Javascript | Brendan Eich      | 1961-07-04 |            |
+------------+-------------------+------------+------------+

See also

 manages decimal precision of floating point operations
(public member function)
changes the width of the next input/output field
(function)
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