I am writing a C++ program to solve a common problem of message decoding. Part of the problem requires me to get a bunch of random characters, including '\', and map them to a key, one by one.
My program works fine in most cases, except that when I read characters such as '\' from a string, I obviously get a completely different character representation (e.g. '\0' yields a null character, or '\' simply escapes itself when it needs to be treated as a character).
Since I am not supposed to have any control on what character keys are included, I have been desperately trying to find a way to treat special control characters such as the backslash as the character itself.
My questions are basically these:
Is there a way to turn all special characters off within the scope of my program?
Is there a way to override current digraphs definitions of special characters and define them as something else (like digraphs using very rare keys)?
Is there some obscure method on the String class that I missed which can force the actual character on the string to be read instead of the pre-defined constant?
I have been trying to look for a solution for hours now but all possible fixes I've found are for other languages.
Any help is greatly appreciate.
If you read in a string like "\0" from stdin or a file, it will be treated as two separate characters: '\\' and '0'. There is no additional processing that you have to do.
Escaping characters is only used for string/character literals. That is to say, when you want to hard-code something into your source code.
Related
I'm really confused about the escape character " \ " and its relation to the windows file system. In the following example:
char* fwdslash = "c:/myfolder/myfile.txt";
char* backslash = "c:\myfolder\myfile.txt";
char* dblbackslash = "c:\\myfolder\\myfile.txt";
std::ifstream file(fwdslash); // Works
std::ifstream file(dblbackslash); // Works
std::ifstream file(backslash); // Doesn't work
I get what you are doing here is escaping a special character so you can use it in this string. In no way by placing a backslash in a string literal or std::string do you actually change the string ---
---Edit: This is completely wrong, and the source of my confusion---
So it seems that the escape character is only treated by certain classes or things to mean something other than a backslash, like outputting on the console, ie., std::cout << "\hello"; will not print the backslash. In the case of ifstream (or I'm not sure if the same applies with the C fopen() version), it must be that this class or function treats backslashes as escape characters. I'm wondering, since the Windows file system uses backslashes wouldn't it make sense for it to accept the simple string with backslashes, ie., "c:\myfolder\myfile.txt" ? Trying it this way fails.
Also, in my compiler (Visual Studio) when I include headers I can use .\ and ..\ to mean either the current folder, or the parent folder. I'm pretty sure the \ in this isn't related to the escape character, but are these forms specific to Windows, part of the C preprocessor, or part of the C or C++ language? I know that backslashes are a Windows thing, so I can't see any reason another system would expect backslashes even when using .\ and ..\
Thanks.
In no way by placing a backslash in a string literal[...] do you
actually change the string
You do. Compiler actually modifies literal you wrote before embedding it into compiled program. If a backslash is found in string or character literal while parsing source code it is ignored and next character is treated specially. \n becomes carriage return, etc. For escaped characters without special meaning threatment is implementation defined. Usually it just means character unchanged.
You cannot just pass "c:\myfolder\file.txt" because it is not a string which will be seen by your program. Your program will see "c:myfolderfile.txt" instead. This is why escaped backslash has a special meaning, to allow embedding backslashes in actual string your program will see.
The solution is to either escape your backslashes, or use raw string literals (C++11 onwards):
const char* path = R"(c:\myfolder\file.txt)"
Filenames given to #include directive are not string literals, even if they are in form "path\to\header", so substitution rules are not applied to them.
The single backwards slash practically escapes the next character. In order to get rid of this behavior you need to double escape it. Now for the forward slash, it is probably a compatibility issue which follows the Unix tradition.
Similar thing to this is also in the Java world. A single forward slash is treated for path separation on both Windows and Unix, while also a double backslash.
To make it more clear why single backslash doesn't work, just remember that the following String practically produces a newline, a backslash and a tab:
"\n\\\t"
i.e. in an example like:
""c:\my\next\file.txt"
would actually produce:
"c:my
ext
ile.txt"
(the double space is form feed, see here)
Because when declaring a cstring literal the backslashes escape the next character, for special characters. This is so you can do newlines (\n), nulls (\0), carriage returns (\r) etc...
char* backslash = "c:\myfolder \myfile.txt";
I'm really confused about the escape character " \ " and its relation to the windows file system. In the following example:
char* fwdslash = "c:/myfolder/myfile.txt";
char* backslash = "c:\myfolder\myfile.txt";
char* dblbackslash = "c:\\myfolder\\myfile.txt";
std::ifstream file(fwdslash); // Works
std::ifstream file(dblbackslash); // Works
std::ifstream file(backslash); // Doesn't work
I get what you are doing here is escaping a special character so you can use it in this string. In no way by placing a backslash in a string literal or std::string do you actually change the string ---
---Edit: This is completely wrong, and the source of my confusion---
So it seems that the escape character is only treated by certain classes or things to mean something other than a backslash, like outputting on the console, ie., std::cout << "\hello"; will not print the backslash. In the case of ifstream (or I'm not sure if the same applies with the C fopen() version), it must be that this class or function treats backslashes as escape characters. I'm wondering, since the Windows file system uses backslashes wouldn't it make sense for it to accept the simple string with backslashes, ie., "c:\myfolder\myfile.txt" ? Trying it this way fails.
Also, in my compiler (Visual Studio) when I include headers I can use .\ and ..\ to mean either the current folder, or the parent folder. I'm pretty sure the \ in this isn't related to the escape character, but are these forms specific to Windows, part of the C preprocessor, or part of the C or C++ language? I know that backslashes are a Windows thing, so I can't see any reason another system would expect backslashes even when using .\ and ..\
Thanks.
In no way by placing a backslash in a string literal[...] do you
actually change the string
You do. Compiler actually modifies literal you wrote before embedding it into compiled program. If a backslash is found in string or character literal while parsing source code it is ignored and next character is treated specially. \n becomes carriage return, etc. For escaped characters without special meaning threatment is implementation defined. Usually it just means character unchanged.
You cannot just pass "c:\myfolder\file.txt" because it is not a string which will be seen by your program. Your program will see "c:myfolderfile.txt" instead. This is why escaped backslash has a special meaning, to allow embedding backslashes in actual string your program will see.
The solution is to either escape your backslashes, or use raw string literals (C++11 onwards):
const char* path = R"(c:\myfolder\file.txt)"
Filenames given to #include directive are not string literals, even if they are in form "path\to\header", so substitution rules are not applied to them.
The single backwards slash practically escapes the next character. In order to get rid of this behavior you need to double escape it. Now for the forward slash, it is probably a compatibility issue which follows the Unix tradition.
Similar thing to this is also in the Java world. A single forward slash is treated for path separation on both Windows and Unix, while also a double backslash.
To make it more clear why single backslash doesn't work, just remember that the following String practically produces a newline, a backslash and a tab:
"\n\\\t"
i.e. in an example like:
""c:\my\next\file.txt"
would actually produce:
"c:my
ext
ile.txt"
(the double space is form feed, see here)
Because when declaring a cstring literal the backslashes escape the next character, for special characters. This is so you can do newlines (\n), nulls (\0), carriage returns (\r) etc...
char* backslash = "c:\myfolder \myfile.txt";
I need to parse some c++ files to get some information out of it. One user case is I have a enum value "ID_XYZ", I want to find out how many times it appears in a source file. So my question is what are the separator dividing symbols in C++?
You can't really tokenize C or C++ source code based purely on separator characters -- you pretty much need to read in a character at a time, and figure out whether that character can be part of the current token or not.
Just for a couple of examples, when you see a C-style begin-comment token, you need to look at characters until you encounter a close-comment token. Likewise, strings and pre-processor directives (e.g., #if 0 .... #endif sequences). To do it truly correctly, you also need to deal correctly with trigraphs. For example, consider something like this:
// Why doesn't this work??/
ID_XYZ = 1;
If the lexer doesn't handle trigraphs correctly, it will probably identify this as an instance of your ID_XYZ -- but in reality, it's not -- the ??/ at the end of the previous line is really a trigraph that resolves to \, which means the "single-line" comment actually extends to the end of the next line, and the apparent instance of ID_XYZ is really part of the comment.
In the program, Lambda λ theoretically represents nothing: ''. I thought of representing this programatically as '\0', but obviously that terminates a string which is not necessarily what lambda does. Also, I am reading in from istringstream and it has problems reading that character in.
So what character would you use?
I'm assuming you have a reason for representing Int,Char,Int as a string, rather than just define a struct to hold the data.
As you say, \0 doesn't work as it terminates the string. But there are other invisible ASCII characters that you can use and easily escape in C++. Have a look at this list of escape codes.
According to the manual, string concatenation isn't implemented in gdb. I need it however, so is there a way to achieve this, perhaps using array functions?
I don't have a copy of gdb around to try this on, but perhaps this line from later in the Ada section of the document will help you?
Rather than use catenation and
symbolic character names to introduce
special characters into strings, one
may instead use a special bracket
notation, which is also used to print
strings. A sequence of characters of
the form ["XX"]' within a string or
character literal denotes the (single)
character whose numeric encoding is XX
in hexadecimal. The sequence of
characters["""]' also denotes a
single quotation mark in strings. For
example, "One line.["0a"]Next
line.["0a"]"
contains an ASCII newline character
(Ada.Characters.Latin_1.LF) after each
period.
For Objective-C:
[#"asd" stringByAppendingString:#"zxc"]
[#"ID: " stringByAppendingString:(NSString*) [aTaskDict valueForKey:#"ID"]]