std::strtof, std::strtod, std::strtold

 

C++

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Strings library

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Null-terminated byte strings

Functions
Character manipulation
isalnum isalpha islower isupper isdigit isxdigit isblank (C++11)	iscntrl isgraph isspace isprint ispunct tolower toupper
Conversions to numeric formats
atof atoiatolatoll strtolstrtoll	strtoulstrtoull strtofstrtodstrtold
String manipulation
strcpy
strncpy
strcat
strncat
strxfrm
String examination
strlen strcmp strncmp strcoll strchr strrchr	strspn strcspn strpbrk strstr strtok
Memory manipulation
memchr
memcmp
memset
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memmove
Miscellaneous
strerror

 

Defined in header <cstdlib>
float       strtof( const char* str, char** str_end );		(since C++11)
double      strtod( const char* str, char** str_end );
long double strtold( const char* str, char** str_end );		(since C++11)

Interprets a floating point value in a byte string pointed to by str.

Function discards any whitespace characters (as determined by std::isspace()) until first non-whitespace character is found. Then it takes as many characters as possible to form a valid floating point representation and converts them to floating point value. The valid floating point value can be one of the following:

• decimal floating point expression. It consists of the following parts:

• (optional) plus or minus sign
• nonempty sequence of decimal digits optionally containing decimal point character (as determined by the current C locale) (defines significand)
• (optional) e or E followed with optional minus or plus sign and nonempty sequence of decimal digits (defines exponent)

• binary floating point expression. It consists of the following parts:

• (optional) plus or minus sign
• 0x or 0X
• nonempty sequence of hexadecimal digits optionally containing a decimal point character (as determined by the current C locale) (defines significand)
• (optional) p or P followed with optional minus or plus sign and nonempty sequence of decimal digits (defines exponent)

• infinity expression. It consists of the following parts:

• (optional) plus or minus sign
• INF or INFINITY ignoring case

• not-a-number expression. It consists of the following parts:

• (optional) plus or minus sign
• NAN or NAN(char_sequence) ignoring case of the NAN part. char_sequence can only contain alphanumeric characters. The result is a quiet NaN floating point value.

• any other expression that may be accepted by the currently installed C locale

The functions sets the pointer pointed to by str_end to point to the character past the last character interpreted. If str_end is NULL, it is ignored.

Parameters

str	-	pointer to the null-terminated byte string to be interpreted
str_end	-	pointer to a pointer to character.

Return value

Floating point value corresponding to the contents of str on success. If the converted value falls out of range of corresponding return type, range error occurs and HUGE_VAL, HUGE_VALF or HUGE_VALL is returned. If no conversion can be performed, ​0​ is returned.

Example

Run this code

#include <cstdlib>
#include <iostream>
 
int main()
{
    std::cout << std::strtod("0.0000000123", nullptr) << "\n";
 
    // the second arg of strto* is non-const. We must copy the string
    char src[] = "0.012  15e16 -0x1afp-2 inF Nan";
    char* c = src;
 
    std::cout << std::strtod(c, &c) << "\n";
    std::cout << std::strtod(c, &c) << "\n";
    std::cout << std::strtod(c, &c) << "\n";
    std::cout << std::strtod(c, &c) << "\n";
    std::cout << std::strtod(c, &c) << "\n";
}

Output:

1.23e-08
0.012
1.5e+17
-107.75
inf
nan

See also

atof	converts a byte string to a floating point value (function)
C documentation for strtof, strtod, strtold