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Re: [std-proposals] Proposal to add f-strings to C++ as an improvement of string formatting

From: Sebastian Wittmeier <wittmeier_at_[hidden]>
Date: Wed, 28 Dec 2022 13:47:06 +0100
Hi Yuri, some quick feedback:   You should best also mention `printf`, even if you perhaps see this as C string formatting. It is available and used on C++.   You destribed f"" / Lf"" format, but did not use the f or Lf specifier in some of the examples in the end.   How would you specify a literal \ as part of the string followed by a {rvalue} expression? \{rvalue} would be interpreted as starting with an escaping \{. Perhaps also escape \ as \\? Then \\{rvalue} could be used for the mentioned output.   The << operator would be a special string language feature. What about >>, e.g. cin >> ? How about translating it into (a variable number of) parameters instead? Which could be further translated into operator chains with templates. To make this approach more flexible, if it would be a language feature.   You have to be more clear about macros. Would it be compatible with the current preprocessor? How would tools fare, that only preprocess C/C++ files for analysis, but do not do further compilation steps. How are the f-strings to be understood as preprocessor tokens?   Is it possible to call functions and lambdas?   Can the {} parsing within the string be done with a compile-time library feature, as soon as there is good reflection support to capture the rvalues? Perhaps there is a more general solution possible?   Best, Sebastian   -----Ursprüngliche Nachricht----- Von:Yuri via Std-Proposals <std-proposals_at_[hidden]> Gesendet:Mi 28.12.2022 07:59 Betreff:[std-proposals] Proposal to add f-strings to C++ as an improvement of string formatting An:std-proposals <std-proposals_at_[hidden]>; CC:Yuri <yuri_at_[hidden]>; ********************************** ** Problem that f-strings solve ** ********************************** String formatting in C++ requires more characters than minimally necessary. There are two major ways to format complex strings in C++: 1. using operator<<, for example:    std::cout << "threshold is " << threshold << ", time is now " << time() << std::endl;        (1) 2. using std::format, for example:    std::cout << std::format("threshold is {}, time is now {}", threshold, time()) << std::endl; (2) Both ways have problems: they require more characters than minimally necessary to perform the task of formatting. The second way also separates variables and their print locations, which makes it more error prone. It's common to need to do a lot of complex message printing for programs, when various values or expressions need to be incorporated into the message. C++ code in these cases is less expressive than it can be. ********************** ** Proposed feature ** ********************** Introduce a special kind of string in C++: f-string. f-string begins with the 'f' character (f"string" or Lf"string"). When an f-string is encountered it is interpreted in the same way as regular strings with this exception: > The '{' character is assumed to begin an rvalue expression. Compiler would > change context after '{' and would begin to parse the rvalue, expecting > to find the closing '}' after the rvalue ends. The closing '}' would restore > the string context. In order for '{' and '}' characters to appear in > an f-string they should be escaped, \{ and \}. Compiler would replace all "...{<rvalue>}..." tokens in f-strings with "..." << <rvalue> << "...", allowing them to be further processed in the same way as an operator<<-based print construct. This way forming complex strings in C++ would become simpler. The above example would look like: std::cout << "threshold is {threshold}, time is now {time()}" << std::endl;                     (3) This is more readable and intuitive than both (1) and (2). ***************** ** Limitations ** ***************** (1) rvalues should be printable with operator<<. (2) for simplicity macros wouldn't be allowed in rvalues embedded in f-strings. ************************************* ** Affected elements of toolchains ** ************************************* * C++ compiler * Syntax highlighters in editors and other software ************** ** Benefits ** ************** f-strings would simplify string formatting and allow to make string formatting code as simple as possible. This would make printing code less error prone, and would make it much easier to read. ***************************** ** What inspired this idea ** ***************************** (1) While writing a lot of C++ code with many complex messages incorporating     a lot of various values I was thinking that it got to be a shorter and     easier way of doing this. (2) While seeing f-strings in Python recently I realized that C++ can also     greatly benefit from this feature. ************* ** Example ** ************* This code would be legal C++ code with f-strings: #include <chrono> #include <ctime> #include <iostream> #include <pwd.h> #include <unistd.h> using namespace std; int main() {         auto now = chrono::system_clock::to_time_t(chrono::system_clock::now());         cout << "Hello {getpwuid(geteuid())->pw_name}"                 ", the time now is {ctime(&now)}." << endl; } Thank you, Yuri Victorovich -- Std-Proposals mailing list Std-Proposals_at_[hidden] https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals

Received on 2022-12-28 12:47:08