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

From cppreference.com
< cpp‎ | string‎ | basic string
m (C++26 P2587R3 changes)
m (→‎Example: +the aforementioned initializer_list header <_<;)
 
(12 intermediate revisions by 6 users not shown)
Line 34: Line 34:
 
Converts a numeric value to {{lc|std::string}}.
 
Converts a numeric value to {{lc|std::string}}.
  
−
{{rrev|until=c++26|
+
{{rrev multi|until1=c++26
 +
|rev1=
 
Let {{tti|buf}} be an internal to the conversion functions buffer, sufficiently large to contain the result of conversion.
 
Let {{tti|buf}} be an internal to the conversion functions buffer, sufficiently large to contain the result of conversion.
  
Line 45: Line 46:
 
@7,8@ Converts a floating point value to a string as if by {{c|std::sprintf(buf, "%f", value)}}.
 
@7,8@ Converts a floating point value to a string as if by {{c|std::sprintf(buf, "%f", value)}}.
 
@9@ Converts a floating point value to a string as if by {{c|std::sprintf(buf, "%Lf", value)}}.
 
@9@ Converts a floating point value to a string as if by {{c|std::sprintf(buf, "%Lf", value)}}.
−
}}
+
|rev2=
−
 
+
−
{{rrev|since=c++26|
+
 
@1-9@ Converts a numeric value to a string as if by {{c|std::format("{}", value)}}.
 
@1-9@ Converts a numeric value to a string as if by {{c|std::format("{}", value)}}.
 
}}
 
}}
Line 63: Line 62:
  
 
===Notes===
 
===Notes===
−
 
 
* With floating point types {{tt|std::to_string}} may yield unexpected results as the number of significant digits in the returned string can be zero, see the example.
 
* With floating point types {{tt|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 {{tt|std::cout}} prints by default, see the example.
 
* The return value may differ significantly from what {{tt|std::cout}} prints by default, see the example.
−
* {{tt|std::to_string}} relies on the current locale for formatting purposes, and therefore concurrent calls to {{tt|std::to_string}} from multiple threads may result in partial serialization of calls. C++17 provides {{ltt|cpp/utility/to_chars|std::to_chars}} as a higher-performance locale-independent alternative.
+
{{rrev|until=c++26|
 +
* {{tt|std::to_string}} relies on the current C locale for formatting purposes, and therefore concurrent calls to {{tt|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.
 +
}}
 +
 
 +
C++17 provides {{ltt|cpp/utility/to_chars|std::to_chars}} as a higher-performance locale-independent alternative.
 +
 
 +
{{feature test macro|__cpp_lib_to_string|Redefining {{tt|std::to_string}} in terms of {{lc|std::format}}|std=C++26|value=202306L}}
  
 
===Example===
 
===Example===
 
{{example
 
{{example
 
|code=
 
|code=
 +
#include <cstdio>
 +
#include <format>
 +
#include <initializer_list>
 
#include <iostream>
 
#include <iostream>
 
#include <string>
 
#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()
 
int main()
 
{
 
{
−
     for (const double f : {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 << "std::cout: " << f << '\n'
+
    {
−
                  << "to_string: " << std::to_string(f) << "\n\n";
+
         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";
 +
    }
 
}
 
}
 +
|p=true
 
|output=
 
|output=
−
std::cout: 23.43
+
to_string:      1.234568 (pre C++26)
−
to_string: 23.430000
+
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
  
−
std::cout: 1e-09
+
to_string:     0.000000 (pre C++26)
−
to_string: 0.000000
+
printf:         0.000000
 +
format:         1e-09
 +
std::cout:      1e-09
  
−
std::cout: 1e+40
+
to_string:     10000000000000000303786028427003666890752.000000 (pre C++26)
−
to_string: 10000000000000000303786028427003666890752.000000
+
printf:         10000000000000000303786028427003666890752.000000
 +
format:         1e+40
 +
std::cout:      1e+40
  
−
std::cout: 1e-40
+
to_string:     0.000000 (pre C++26)
−
to_string: 0.000000
+
printf:         0.000000
 +
format:         1e-40
 +
std::cout:      1e-40
  
−
std::cout: 1.23457e+08
+
to_string:      123456789.000000 (pre C++26)
−
to_string: 123456789.000000
+
printf:        123456789.000000
 +
format:        123456789
 +
std::cout:     1.23457e+08
 
}}
 
}}
  

Latest revision as of 12:29, 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

[edit] Parameters

value - a numeric value to convert

[edit] Return value

A string holding the converted value.

[edit] Exceptions

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

[edit] 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

[edit] Example

Run this code
#include <cstdio>
#include <format>
#include <initializer_list>
#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

[edit] 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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