if-statement on different typeid's - c++

I am currently working on a code and I am trying to use a if-statement on a variable which was taken from a .txt file with a basic string. Its supposed to look like
if (a.variable == "string") {}
When I use
std::cout << a.variable << std::endl;
std::cout << "string" << std::endl;
I get the same results but when using
std::cout << typeid(a.variable).name() << std::endl;
std::cout << typeid("string").name() << std::endl;
I get different results:
NSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE
and
A5_c.
Could this be the reason why the if-statement failed? Unless I am incorrect, the first typeid stands for a basic string.
I am grateful for any input!
The code I use for reading it looks like:
std::string::size_type beginoption = section.find("=",position);
beginoption = beginoption +1;
std::string::size_type endoption = section.find("\n",position);
optionstorage = section.substr(beginoption, endoption - beginoption);

Two objects in C++ don't have to be the same type to compare as equal. You can compare string objects to string literals because there is an operator== overload that accepts std::string and char const * arguments. (The typeid() operator returns a different value because the two expressions have different types; one is a string object and the other is a char array -- but you can indeed still compare them.)
You mentioned that your "if statement is failing" but when you inspect the contents of the strings, they appear to be the same -- they may actually not be the same. For example, in your code, if a.variable has trailing whitespace, you would not see this in the output and yet the strings would also not be equal.
Try writing both strings surrounded by some characters. I suspect that you will see there is some extra whitespace somewhere:
std::cout << '[' << a.variable << ']' << std::endl;
std::cout << '[' << "string" << ']' << std::endl;
Consider also displaying a.variable.size(). If it's not 6, then the two strings cannot be equal since they have different lengths.

For the purposes of typeid, a.variable is of type std::string while the string literal "string" is of type char const [7].
That explains the output of
std::cout << typeid(a.variable).name() << std::endl;
std::cout << typeid("string").name() << std::endl;

Related

How to display character string literals with hex properly with std::cout in C++?

How to display character string literals with hex properly with std::cout in C++?
I want to use octal and hex to print character string literals with std::cout in C++.
I want to print "bee".
#include <iostream>
int main() {
std::cout << "b\145e" << std::endl;//1
std::cout << "b\x65e" << std::endl;//2
return 0;
}
//1 works fine, but //2 doesn't with hex escape sequence out of range.
Now I want to print "be3".
#include <iostream>
int main() {
std::cout << "b\1453" << std::endl;//1
std::cout << "b\x653" << std::endl;//2
return 0;
}
Also, //1 works fine, but //2 doesn't with hex escape sequence out of range.
Now can I come to the conclusion that hex is not a good way to display character string characters?
I get the feeling I am wrong but don't know why.
Can someone explain whether hex can be used and how?
There's actually an example of this exact same situation on cppreference's documentation on string literals.
If a valid hex digit follows a hex escape in a string literal, it would fail to compile as an invalid escape sequence. String concatenation can be used as a workaround:
They provide the example below:
// const char* p = "\xfff"; // error: hex escape sequence out of range
const char* p = "\xff""f"; // OK : the literal is const char[3] holding {'\xff','f','\0'}
Applying what they explain to your problem, we can print the string literal be3 in two ways:
std::cout << "b\x65" "3" << std::endl;
std::cout << "b\x65" << "3" << std::endl;
The hex escape sequences becomes \x65e and \x653 so you need to help the compiler to stop after 65:
#include <iostream>
int main() {
std::cout << "b\x65""e" << std::endl;//2
std::cout << "b\x65""3" << std::endl;//2
}

C++ single quotes syntax

I am learning C++ and just started reading "Programming Principles and Practice" by Bjarne Stroustrup and he uses this code to illustrate a point:
#include "std_lib_facilities.h"
using namespace std;
int main() // C++ programs start by executing the function main
{
char c = 'x';
int i1 = c;
int i2 = 'x';
char c2 = i1;
cout << c << ' << i1 << ' << c2 << '\n';
return 0;
}
I am familiar in general with the difference between double and single quotes in the C++ world, but would someone kindly explain the construction and purpose of the section ' << i1 << '
Thanks
cout << c << ' << i1 << ' << c2 << '\n';
appears to be a typo in the book. I see it in Programming Principles and Practice Using C++ (Second Edition) Second printing. I do not see it listed in the errata.
According to the book, the intended output is
x 120 x
But what happens here is ' << i1 << ' attempts to compress the << i1 << to a multi-byte character and prints out an integer (most likely 540818464-> 0x203C3C20 -> ASCII values of ' ', '<', '<', ' ') because cout doesn't know wide characters. You'd need wcout for that. End result is output something like
x540818464x
and a warning or two from the compiler because while it's valid C++ code, it's almost certainly not what you want to be doing.
The line should most likely read
cout << c << ' ' << i1 << ' ' << c2 << '\n';
which will output the expected x 120 x
In other words, Linker3000, you are not crazy and not misunderstanding the example code.
Anyone know who I should contact to log errata or get a clarification on the off chance there is some top secret sneakiness going way over my head?
Before answering your question, here is a little background on what that is actually doing. Also note that there is a typo in the example, the string constant should have been double quoted:
cout << c << " << i1 << " << c2 << "\n";
In C++, operators can be overloaded so that they mean different things with different functions. In the case of cout, the << operator is overloaded as the "Insertion Operator". Think of it as taking the operand on the right, and inserting it (or sending it) into the operator on the left.
For example,
cout << "Hello World";
This takes the string "Hello World", and sends it to cout for processing.
So what beginners do not get is what something like this means:
cout << "Hello" << " World";
This is doing the same thing, but the operator precedence says to perform the injections from left to right. To make this work, the cout object returns itself as a function return value. Why is this important? Because the above statement is actually two separate operator evaluations:
(cout << "Hello") << " World";
This first injects "Hello" to cout, which outputs it, then continues to evaluate the next inject operator. Because cout returns itself, after the (cout << "Hello") is executed you have the following still to be evaluated:
cout << " World";
This expression injects " World" into the cout object, which then outputs " World", with the net effect being that you see "Hello World" just like the first time.
So in your example, what is it doing?
cout << c << " << i1 << " << c2 << "\n";
This is evaluated left to right as follows:
((((cout << c) << " << i1 << ") << c2) << "\n"); => Outputs value of c
((((cout ) << " << i1 << ") << c2) << "\n"); => Outputs string " << i1 << "
((( cout ) << c2) << "\n"); => Outputs value of c2
(( cout ) << "\n"); => Outputs newline character
( cout ); => No more output
Expression completes and returns the cout object as the expression value.
Assuming c='x' and c2='x', the final output from this expression is the following character string output on a single line:
x << i1 << x
For beginners, all those insertion operators << look a little strange. It is because you are dealing with objects. You could build the string up as a complete formatted object before injecting it into cout, and while that make the cout expression look simpler, we do not do that in C++ because it makes your code more complex and error prone. Note also, there is nothing special about the cout object. If you wanted to output to the standard error stream, you would use cerr instead. If you wanted to output to a file, your would instantiate a stream object that outputs to the desired file. That rest of the code in your example would be the same.
In C, the same thing would be done procedurally using a format string:
printf("%d << i1 << %d\n", i1, c2);
This is allowed in C++ too, because C++ is a superset of C. Many C++ programmers still use this output method, but that is because those programmers learned C first, and may not have fully embraced the object oriented nature of C++
Note that you may also have seen the << operator in the context of mathematical expressions like:
A = A << 8;
In this case, the << operator is the bitwise rotate operation. It has nothing to do with output to cout. It will rotate the bits in A to the left by eight bits.

C++ substr method - "invalid use of ‘this’ in non-member function"

I tried to compile the following code
std::string key = "DISTRIB_DESCRIPTION=";
std::cout << "last five characters: " << key.substr(this.end()-5) << '\n';
And the compiler says
error: invalid use of ‘this’ in non-member function
std::cout << "last five characters: " << key.substr(this.end()-5) << '\n';
^
substr is a "public member function" of std::string, why can't I use this?
I know I could just reference key again instead of this, but my original code was
std::cout << "Description: " << line.substr(found+key.length()+1).substr(this.begin(),this.length()-1) << '\n';
In the second use of substr, the string does not have a name, so the only way to refer to it would be this. I fixed it with
std::cout << "Description: " << line.substr(found+key.length()+1,line.length()-found-key.length()-2) << '\n';
But I am now curious to why this won't work.
this is only available when you are writing code as part of a non-static method of a class. In your particular case, it seems obvious to you that this should refer to key, but the compiler sees no reason for that.
Also, string.substr() takes an integer indicating the beginning position. string.end() returns an iterator, which will not work. What you likely want to do here is call string.length().
Simply replace the first piece of code with:
std::cout << "last five characters: " << key.substr(key.length()-5) << '\n';
And you should be okay.

Why setting null in the middle of std string doesn't have any effect

Consider
#include <string>
#include <iostream>
int main()
{
/*
hello
5
hel
3
*/
char a[] = "hello";
std::cout << a << std::endl;
std::cout << strlen(a) << std::endl;
a[3] = 0;
std::cout << a << std::endl;
std::cout << strlen(a) << std::endl;
/*
hello
5
hel o
5
*/
std::string b = "hello";
std::cout << b << std::endl;
std::cout << b.length() << std::endl;
b[3] = 0;
std::cout << b << std::endl;
std::cout << b.length() << std::endl;
getchar();
}
I expect std::string will behave identical to char array a. That's it, insert null character in the middle of the string, will "terminate" the string. However, it is not the case. Is my expectation wrong?
A std::string is not like a usual C string, and can contain embedded NUL characters without problems. However, if you do this you will notice the string is prematurely terminated if you use the .c_str() function to return a const char *.
No - std::strings are not NUL-terminated like C "strings"; the std::string records its length independently.
#Lou is right: don't do that. Instead, do this:
b.erase (3, b.length());
Yes, your expectation is wrong. std::string is meant to be different from C strings (e.g. not necessarily stored in consecutive memory / an array).
To duplicate the first section's behavior, try std::cout << b.c_str() instead of std::cout << b.
I expect std::string will behave identical to char array a.
Why? Nothing in the documentation, anywhere, having to do with std::string says it does this.
My suggestion, stop treating like C++ as C plus some stuff.

Comparing Character Literal to Std::String in C++

I would like to compare a character literal with the first element of string, to check for comments in a file. Why use a char? I want to make this into a function, which accepts a character var for the comment. I don't want to allow a string because I want to limit it to a single character in length.
With that in mind I assumed the easy way to go would be to address the character and pass it to the std::string's compare function. However this is giving me unintended results.
My code is as follows:
#include <string>
#include <iostream>
int main ( int argc, char *argv[] )
{
std::string my_string = "bob";
char my_char1 = 'a';
char my_char2 = 'b';
std::cout << "STRING : " << my_string.substr(0,1) << std::endl
<< "CHAR : " << my_char1 << std::endl;
if (my_string.substr(0,1).compare(&my_char1)==0)
std::cout << "WOW!" << std::endl;
else
std::cout << "NOPE..." << std::endl;
std::cout << "STRING : " << my_string.substr(0,1) << std::endl
<< "CHAR : " << my_char2 << std::endl;
if (my_string.substr(0,1).compare(&my_char2)==0)
std::cout << "WOW!" << std::endl;
else
std::cout << "NOPE..." << std::endl;
std::cout << "STRING : " << my_string << std::endl
<< "STRING 2 : " << "bob" << std::endl;
if (my_string.compare("bob")==0)
std::cout << "WOW!" << std::endl;
else
std::cout << "NOPE..." << std::endl;
}
Gives me...
STRING : b
CHAR : a
NOPE...
STRING : b
CHAR : b
NOPE...
STRING : bob
STRING 2 : bob
WOW!
Why does the function think the sub-string and character aren't the same. What's the shortest way to properly compare chars and std::string vars?
(a short rant to avoid reclassification of my question.... feel free to skip)
When I say shortest I mean that out of a desire for coding eloquence. Please note, this is NOT a homework question. I am a chemical engineering Ph.D candidate and am coding as part of independent research. One of my last questions was reclassified as "homework" by user msw (who also made a snide remark) when I asked about efficiency, which I considered on the border of abuse. My code may or may not be reused by others, but I'm trying to make it easy to read and maintainable. I also have a bizarre desire to make my code as efficient as possible where possible. Hence the questions on efficiency and eloquence.
Doing this:
if (my_string.substr(0,1).compare(&my_char2)==0)
Won't work because you're "tricking" the string into thinking it's getting a pointer to a null-terminated C-string. This will have weird effects up to and including crashing your program. Instead, just use normal equality to compare the first character of the string with my_char:
if (my_string[0] == my_char)
// do stuff
Why not just use the indexing operator on your string? It will return a char type.
if (my_string[0] == my_char1)
You can use the operator[] of string to compare it to a single char
// string::operator[]
#include <iostream>
#include <string>
using namespace std;
int main ()
{
string str ("Test string");
int i; char c = 't';
for (i=0; i < str.length(); i++)
{
if (c == str[i]) {
std::cout << "Equal at position i = " << i << std::endl;
}
}
return 0;
}
The behaviour of the first two calls to compare is entirely dependent on what random memory contents follows the address of each char. You are calling basic_string::compare(const char*) and the param here is assumed to be a C-String (null-terminated), not a single char. The compare() call will compare your desired char, followed by everything in memory after that char up to the next 0x00 byte, with the std::string in hand.
Otoh the << operator does have a proper overload for char input so your output does not reflect what you are actually comparing here.
Convert the decls of and b to be const char[] a = "a"; and you will get what you want to happen.
Pretty standard, strings in c++ are null-terminated; characters are not. So by using the standard compare method you're really checking if "b\0" == 'b'.
I used this and got the desired output:
if (my_string.substr(0,1).compare( 0, 1, &my_char2, 1)==0 )
std::cout << "WOW!" << std::endl;
else
std::cout << "NOPE..." << std::endl;
What this is saying is start at position 0 of the substring, use a length of 1, and compare it to my character reference with a length of 1. Reference