In addition to many beneficial features of C++ 17, there are elementary string conversions introduced in that specification. The std::to_chars() and std::from_chars() are defined in <charconv> header to do conversions between numeric values to strings or strings to numeric values without considering locale-specific conversions. In this post, we explain the std::to_chars() and std::from_chars() that come with C++17.
Table of Contents
What Are The Elementary String Conversions That Come With C++ 17 ?
What is std::to_chars()?
The std::to_chars() is defined in header and is used to convert numeric values into a character string within the given valid range.
The std::to_chars() is designed to copy the numeric value into a string buffer, with a given specific format, with the only overhead of making sure that the buffer is big enough. You don’t need to consider locale-specific conversions.
Here is the syntax of std::to_chars in C++ 17.
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std::to_chars_result to_chars( char* first, char* last, value, int base = 10 ); |
Here is a simple example.
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std::string str= "abcdefgh"; const int ival = 10001000; const auto con = std::to_chars( str.data(), str.data() + str.size(), ival); |
In floating point number conversions, std::chars_format types can be used (i.e. std::chars_format::fixed, std::chars_format::scientific, std::chars_format::general,…)
What is std::from_chars()?
The std::from_chars() is defined in the header and used to convert the string data in a given range to a value (string to int, string to float operations) if no string characters match the pattern or if the obtained value is not representable in a given type of value, then the value has remains unchanged.
The std::from_chars() is a lightweight parser that does not need to create dynamic allocation, and you don’t need to consider locale-specific conversions.
Here is the syntax of std::from_chars in C++ 17.
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std::from_chars_result from_chars( const char* first, const char* last, &value, int base = 10 ); |
Here is a simple example.
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std::string str= "10001000"; int vali; auto [ptr, ec] = std::from_chars(str.data(), str.data() + str.size(), vali); |
Is there a full example to elementary string conversions that comes with C++ 17?
Here is a full example about std::to_chars() and std::from_chars() in C++ 17.
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#include <iostream> #include <string> #include <charconv> int main() { std::string str= "abcdefgh"; // INT TO CHAR const int ival = 10001000; const auto con = std::to_chars( str.data(), str.data() + str.size(), ival); std::cout << str << std::endl; // FLOAT TO CHAR std::string str2= "abcdefgh"; const float fval = 9.81f; const auto con2 = std::to_chars( str2.data(), str2.data() + str2.size(), fval, std::chars_format::fixed, 6); std::cout << str2 << std::endl; // CHAR TO INT int vali; auto [ptr, ec] = std::from_chars(str.data(), str.data() + str.size(), vali); std::cout << vali << std::endl; // CHAR TO FLOAT float valf; auto [ptr2, ec2] = std::from_chars(str2.data(), str2.data() + str2.size(), valf); std::cout << valf << std::endl; system("pause"); return 0; } |
Here is the output.
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10001000 9.810000 10001000 9.81 |
Note that here ec is error check parameter and can be used to check error by if( ec = std::erc() ) { ... } or other error check members can be used, i.e. if (ec == std::errc::invalid_argument) { ... }. The benefits of these conversion methods are compatible with the locale-specific conversions.
For more details about this feature in C++17 standard, please see these papers; P0067R5, P0682R1DR
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