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Difference between revisions of "cpp/string/basic string/to string"

From cppreference.com
< cpp‎ | string‎ | basic string
m (→‎Example: ~)
m (→‎Example: fmt.)
Line 81: Line 81:
 
#include <string>
 
#include <string>
  
−
const char* const to_string_label =
 
 
#if __cpp_lib_to_string >= 202306L
 
#if __cpp_lib_to_string >= 202306L
−
    " (post C++26)";
+
constexpr auto revision() { return " (post C++26)"; }
 
#else
 
#else
−
    " (pre C++26)";
+
constexpr auto revision() { return " (pre C++26)"; }
 
#endif
 
#endif
  
Line 92: Line 91:
 
     for (const double f : {1.23456789555555, 23.43, 1e-9, 1e40, 1e-40, 123456789.0})
 
     for (const double f : {1.23456789555555, 23.43, 1e-9, 1e40, 1e-40, 123456789.0})
 
     {
 
     {
−
         std::cout << "to_string: " << std::to_string(f) << to_string_label << '\n';
+
         std::cout << "to_string:\t" << std::to_string(f) << revision() << '\n';
  
 
         // Before C++26, the output of std::to_string matches std::printf.
 
         // Before C++26, the output of std::to_string matches std::printf.
−
         std::printf("printf:   %f\n", f);
+
         std::printf("printf:\t\t%f\n", f);
  
 
         // As of C++26, the output of std::to_string matches std::format.
 
         // As of C++26, the output of std::to_string matches std::format.
−
         std::cout << std::format("format:   {}\n", f);
+
         std::cout << std::format("format:\t\t{}\n", f);
  
−
         std::cout << "std::cout: " << f << "\n\n";
+
         std::cout << "std::cout:\t" << f << "\n\n";
 
     }
 
     }
 
}
 
}
 
|p=true
 
|p=true
 
|output=
 
|output=
−
to_string: 1.234568 (pre C++26)
+
to_string:     1.234568 (pre C++26)
−
printf:   1.234568
+
printf:         1.234568
−
format:   1.23456789555555
+
format:         1.23456789555555
−
std::cout: 1.23457
+
std::cout:     1.23457
  
−
to_string: 23.430000 (pre C++26)
+
to_string:     23.430000 (pre C++26)
−
printf:   23.430000
+
printf:         23.430000
−
format:   23.43
+
format:         23.43
−
std::cout: 23.43
+
std::cout:     23.43
  
−
to_string: 0.000000 (pre C++26)
+
to_string:     0.000000 (pre C++26)
−
printf:   0.000000
+
printf:         0.000000
−
format:   1e-09
+
format:         1e-09
−
std::cout: 1e-09
+
std::cout:     1e-09
  
−
to_string: 10000000000000000303786028427003666890752.000000 (pre C++26)
+
to_string:     10000000000000000303786028427003666890752.000000 (pre C++26)
−
printf:   10000000000000000303786028427003666890752.000000
+
printf:         10000000000000000303786028427003666890752.000000
−
format:   1e+40
+
format:         1e+40
−
std::cout: 1e+40
+
std::cout:     1e+40
  
−
to_string: 0.000000 (pre C++26)
+
to_string:     0.000000 (pre C++26)
−
printf:   0.000000
+
printf:         0.000000
−
format:   1e-40
+
format:         1e-40
−
std::cout: 1e-40
+
std::cout:     1e-40
  
−
to_string: 123456789.000000 (pre C++26)
+
to_string:     123456789.000000 (pre C++26)
−
printf:   123456789.000000
+
printf:         123456789.000000
−
format:   123456789
+
format:         123456789
−
std::cout: 1.23457e+08
+
std::cout:     1.23457e+08
 
}}
 
}}
  

Revision as of 11:58, 16 May 2024

 
C++
 
Strings library
 
std::basic_string
Member functions
(C++23)
Element access
(DR*)
(DR*)
(C++17)
Iterators
(C++11)
(C++11)
(C++11)
(C++11)
Capacity
(DR*)
Modifiers
(C++23)
(DR*)
(C++23)
(C++23)
(C++23)  
Search
Operations
(C++20)
(C++20)
(C++23)
Constants
Non-member functions
(C++20)(C++20)
I/O
Comparison
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)
Numeric conversions
(C++11)(C++11)(C++11)
(C++11)(C++11)
(C++11)(C++11)(C++11)
to_string
(C++11)
(C++11)
Literals
(C++14)
Helper classes
(C++11)
Deduction guides (C++17)

 
Defined in header <string>
std::string to_string( int value );
 (1) (since C++11)
std::string to_string( long value );
 (2) (since C++11)
std::string to_string( long long value );
 (3) (since C++11)
std::string to_string( unsigned value );
 (4) (since C++11)
std::string to_string( unsigned long value );
 (5) (since C++11)
std::string to_string( unsigned long long value );
 (6) (since C++11)
std::string to_string( float value );
 (7) (since C++11)
std::string to_string( double value );
 (8) (since C++11)
std::string to_string( long double value );
 (9) (since C++11)

Converts a numeric value to std::string.

Let buf be an internal to the conversion functions buffer, sufficiently large to contain the result of conversion.

1) Converts a signed integer to a string as if by std::sprintf(buf, "%d", value).
2) Converts a signed integer to a string as if by std::sprintf(buf, "%ld", value).
3) Converts a signed integer to a string as if by std::sprintf(buf, "%lld", value).
4) Converts an unsigned integer to a string as if by std::sprintf(buf, "%u", value).
5) Converts an unsigned integer to a string as if by std::sprintf(buf, "%lu", value).
6) Converts an unsigned integer to a string as if by std::sprintf(buf, "%llu", value).
7,8) Converts a floating point value to a string as if by std::sprintf(buf, "%f", value).
9) Converts a floating point value to a string as if by std::sprintf(buf, "%Lf", value).
(until C++26)
1-9) Converts a numeric value to a string as if by std::format("{}", value).
(since C++26)

Contents

Parameters

value - a numeric value to convert

Return value

A string holding the converted value.

Exceptions

May throw std::bad_alloc from the std::string constructor.

Notes

  • With floating point types std::to_string may yield unexpected results as the number of significant digits in the returned string can be zero, see the example.
  • The return value may differ significantly from what std::cout prints by default, see the example.
  • std::to_string relies on the current C locale for formatting purposes, and therefore concurrent calls to std::to_string from multiple threads may result in partial serialization of calls.
    • The results of overloads for integer types do not rely on the current C locale, and thus implementations generally avoid access to the current C locale in these overloads for both correctness and performance. However, such avoidance is not guaranteed by the standard.
(until C++26)

C++17 provides std::to_chars as a higher-performance locale-independent alternative.

Feature-test macro Value Std Feature
__cpp_lib_to_string 202306L (C++26) Redefining std::to_string in terms of std::format

Example

Run this code
#include <cstdio>
#include <format>
#include <iostream>
#include <string>
 
#if __cpp_lib_to_string >= 202306L
constexpr auto revision() { return " (post C++26)"; }
#else
constexpr auto revision() { return " (pre C++26)"; }
#endif
 
int main()
{
    for (const double f : {1.23456789555555, 23.43, 1e-9, 1e40, 1e-40, 123456789.0})
    {
        std::cout << "to_string:\t" << std::to_string(f) << revision() << '\n';
 
        // Before C++26, the output of std::to_string matches std::printf.
        std::printf("printf:\t\t%f\n", f);
 
        // As of C++26, the output of std::to_string matches std::format.
        std::cout << std::format("format:\t\t{}\n", f);
 
        std::cout << "std::cout:\t" << f << "\n\n";
    }
}

Possible output: 

to_string:      1.234568 (pre C++26)
printf:         1.234568
format:         1.23456789555555
std::cout:      1.23457
 
to_string:      23.430000 (pre C++26)
printf:         23.430000
format:         23.43
std::cout:      23.43
 
to_string:      0.000000 (pre C++26)
printf:         0.000000
format:         1e-09
std::cout:      1e-09
 
to_string:      10000000000000000303786028427003666890752.000000 (pre C++26)
printf:         10000000000000000303786028427003666890752.000000
format:         1e+40
std::cout:      1e+40
 
to_string:      0.000000 (pre C++26)
printf:         0.000000
format:         1e-40
std::cout:      1e-40
 
to_string:      123456789.000000 (pre C++26)
printf:         123456789.000000
format:         123456789
std::cout:      1.23457e+08

See also

(C++11)
 converts an integral or floating-point value to wstring
(function) [edit]
(C++11)(C++11)
 converts a string to an unsigned integer
(function) [edit]
(C++11)(C++11)(C++11)
 converts a string to a signed integer
(function) [edit]
(C++11)(C++11)(C++11)
 converts a string to a floating point value
(function) [edit]
(C++17)
 converts an integer or floating-point value to a character sequence
(function) [edit]
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