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digitalmars.D.learn - Is there an opposite of .toString()?

reply myst <a b.c> writes:
I'm sorry if this has been answered already, it seems like a very 
basic question.

There is .toString() method convention for printing, but I can 
not find anything alike for reading. Is there something like 
operator>>() in C++? What's an ideomatic way of reading an object?
Oct 13
next sibling parent reply Fra Mecca <me francescomecca.eu> writes:
On Saturday, 14 October 2017 at 00:18:35 UTC, myst wrote:
 I'm sorry if this has been answered already, it seems like a 
 very basic question.

 There is .toString() method convention for printing, but I can 
 not find anything alike for reading. Is there something like 
 operator>>() in C++? What's an ideomatic way of reading an 
 object?
Can you make an example of "reading from object"? Or better some snippet of a c++ code that does what you mean
Oct 13
parent myst <a b.c> writes:
On Saturday, 14 October 2017 at 02:16:12 UTC, Fra Mecca wrote:
 On Saturday, 14 October 2017 at 00:18:35 UTC, myst wrote:
 ...
 some snippet of a c++ code that does what you mean
#include <istream> #include <string> struct S { std::string s; int p; int n; }; std::istream& operator>>(std::istream& is, S& s) { is >> s.s >> s.p >> s.n; return is; }
Oct 13
prev sibling parent reply Jonathan M Davis <newsgroup.d jmdavisprog.com> writes:
On Saturday, October 14, 2017 00:18:35 myst via Digitalmars-d-learn wrote:
 I'm sorry if this has been answered already, it seems like a very
 basic question.

 There is .toString() method convention for printing, but I can
 not find anything alike for reading. Is there something like
 operator>>() in C++? What's an ideomatic way of reading an object?
The function to use for conversions in general is std.conv.to. And really, there isn't much of a reason to ever call toString. Functions like writeln, format, and to may use it internally, but it's more or less an anti-pattern to do so in your own code - especially if we're talking about generic code. If you're looking to convert something to string, to!string works with pretty much everything and toString works with considerably less. And if there's a generic way to convert from string to something else, it's also with to - e.g. to!int("42"). However, for that conversion to work, it either has to be a built-in type so that to understands it, or the type will need a constructor that takes a string. In general, in order to generically convert to a user-defined type, either that target type must have a constructor that accepts that source type, or the source type must define opCast or an alias to convert to the target type. std.conv.to is very powerful, but it does need to have something to work with. If anything approaching a standard conversion exists, it can be done with std.conv.to; otherwise, it's going to depend on the type. I think that in general, you're going to find that converting to a string works with most everything, but aside from built-in types, converting from a string with std.conv.to is unlikely to work. _Some_ types do have constructors that take strings, but most don't. Built-in types will work, because std.conv.to understands how to do that conversion. For user-defined types, either you're likely going to have to parse the string yourself, or they may contain another function for doing the conversion (for instance std.datetime.systime.SysTime uses toISOExtString and fromISOExtString to convert to and from the ISO extended format for a date and time and has other functions for other time formats). You can also check out std.conv.parse, which acts similarly to std.conv.to, but whereas to converts the entire string, parse converts the first portion of a string and therefore is meant to allow for parsing multiple values from a string. - Jonathan M Davis
Oct 13
next sibling parent myst <a b.c> writes:
On Saturday, 14 October 2017 at 03:47:29 UTC, Jonathan M Davis 
wrote:
 The function to use for conversions in general is std.conv.to. 
 And really, there isn't much of a reason to ever call toString. 
 Functions like writeln, format, and to may use it internally, 
 but it's more or less an anti-pattern to do so in your own code 
 - especially if we're talking about generic code. If you're 
 looking to convert something to string, to!string works with 
 pretty much everything and toString works with considerably 
 less. And if there's a generic way to convert from string to 
 something else, it's also with to - e.g. to!int("42"). However, 
 for that conversion to work, it either has to be a built-in 
 type so that to understands it, or the type will need a 
 constructor that takes a string. In general, in order to 
 generically convert to a user-defined type, either that target 
 type must have a constructor that accepts that source type, or 
 the source type must define opCast or an alias to convert to 
 the target type. std.conv.to is very powerful, but it does need 
 to have something to work with. If anything approaching a 
 standard conversion exists, it can be done with std.conv.to; 
 otherwise, it's going to depend on the type.

 I think that in general, you're going to find that converting 
 to a string works with most everything, but aside from built-in 
 types, converting from a string with std.conv.to is unlikely to 
 work. _Some_ types do have constructors that take strings, but 
 most don't. Built-in types will work, because std.conv.to 
 understands how to do that conversion. For user-defined types, 
 either you're likely going to have to parse the string 
 yourself, or they may contain another function for doing the 
 conversion (for instance std.datetime.systime.SysTime uses 
 toISOExtString and fromISOExtString to convert to and from the 
 ISO extended format for a date and time and has other functions 
 for other time formats).
This is not what I asked about.
 You can also check out std.conv.parse, which acts similarly to 
 std.conv.to, but whereas to converts the entire string, parse 
 converts the first portion of a string and therefore is meant 
 to allow for parsing multiple values from a string.
This is what I asked about. Thanks.
Oct 14
prev sibling parent Faux Amis <faux amis.com> writes:
On 2017-10-14 05:47, Jonathan M Davis wrote:
 On Saturday, October 14, 2017 00:18:35 myst via Digitalmars-d-learn wrote:
 I'm sorry if this has been answered already, it seems like a very
 basic question.

 There is .toString() method convention for printing, but I can
 not find anything alike for reading. Is there something like
 operator>>() in C++? What's an ideomatic way of reading an object?
The function to use for conversions in general is std.conv.to. And really, there isn't much of a reason to ever call toString. Functions like writeln, format, and to may use it internally, but it's more or less an anti-pattern to do so in your own code - especially if we're talking about generic code. If you're looking to convert something to string, to!string works with pretty much everything and toString works with considerably less. And if there's a generic way to convert from string to something else, it's also with to - e.g. to!int("42"). However, for that conversion to work, it either has to be a built-in type so that to understands it, or the type will need a constructor that takes a string. In general, in order to generically convert to a user-defined type, either that target type must have a constructor that accepts that source type, or the source type must define opCast or an alias to convert to the target type. std.conv.to is very powerful, but it does need to have something to work with. If anything approaching a standard conversion exists, it can be done with std.conv.to; otherwise, it's going to depend on the type. I think that in general, you're going to find that converting to a string works with most everything, but aside from built-in types, converting from a string with std.conv.to is unlikely to work. _Some_ types do have constructors that take strings, but most don't. Built-in types will work, because std.conv.to understands how to do that conversion. For user-defined types, either you're likely going to have to parse the string yourself, or they may contain another function for doing the conversion (for instance std.datetime.systime.SysTime uses toISOExtString and fromISOExtString to convert to and from the ISO extended format for a date and time and has other functions for other time formats). You can also check out std.conv.parse, which acts similarly to std.conv.to, but whereas to converts the entire string, parse converts the first portion of a string and therefore is meant to allow for parsing multiple values from a string. - Jonathan M Davis
Thanks for the proper/complete reply ;)
Oct 15