map<string, int> M;
for (auto E: M)
{
cout << E.first << ": " << E.second << endl;
F << E.first << ": " << E.second << endl;
};
I am learning c++ and I am confused with auto. I am trying to convert the above code into the following code( the above code with auto works correctly)
map<string, int> M;
for (map<string, int> :: iterator p = begin(M); p != end(M); p ++ )
{
cout << p.first << ": " << p.second << endl;
F << p.first << ": " << p.second << endl;
}
I got the following error:
error: ‘std::map<std::basic_string<char>, int>::iterator’ has no member named ‘first’
cout << p.first << ": " << p.second << endl;
error: ‘std::map<std::basic_string<char>, int>::iterator’ has no member named ‘second’
cout << p.first << ": " << p.second << endl;
error: ‘std::map<std::basic_string<char>, int>::iterator’ has no member named ‘first’
F << p.first << ": " << p.second << endl;
error: ‘std::map<std::basic_string<char>, int>::iterator’ has no member named ‘second’
F << p.first << ": " << p.second << endl;
Why it does not work?
Iterators are like pointers and must be dereferenced for use:
cout << p->first << ": " << p->second << endl;
The ranged-for loop (the example with auto) did this for you.
Related
I have following test example:
#include <iostream>
#include <vector>
void foo (std::vector<int> value) {
std::cout << "value "
<< &value
<< " "
<< value.data()
<< " "
<< value.size()
<< std::endl;
}
void foo2 (std::vector<int>&& rvalure_ref) {
std::cout << "rvalue_ref "
<< &rvalure_ref
<< " "
<< rvalure_ref.data()
<< " "
<< rvalure_ref.size()
<< std::endl;
}
int main() {
std::vector<int> value(5, 0);
std::cout << "init "
<< &value
<< " "
<< value.data()
<< " "
<< value.size()
<< std::endl;
foo(std::move(value));
std::cout << "done "
<< &value
<< " "
<< value.data()
<< " "
<< value.size()
<< std::endl;
}
The result of the code above is:
init 0x7ffed27c6450 0x56480bc1eeb0 5
value 0x7ffed27c6470 0x56480bc1eeb0 5
done 0x7ffed27c6450 0 0
Looks great:
Now, move to:
#include <iostream>
#include <vector>
void foo (std::vector<int> value) {
std::cout << "value "
<< &value
<< " "
<< value.data()
<< " "
<< value.size()
<< std::endl;
}
void foo2 (std::vector<int>&& rvalure_ref) {
std::cout << "rvalue_ref "
<< &rvalure_ref
<< " "
<< rvalure_ref.data()
<< " "
<< rvalure_ref.size()
<< std::endl;
}
int main() {
std::vector<int> value(5, 0);
std::cout << "init "
<< &value
<< " "
<< value.data()
<< " "
<< value.size()
<< std::endl;
foo2(std::move(value));
std::cout << "done "
<< &value
<< " "
<< value.data()
<< " "
<< value.size()
<< std::endl;
}
The result is:
init 0x7ffccc93a5c0 0x56124b3a8eb0 5
rvalue_ref 0x7ffccc93a5c0 0x56124b3a8eb0 5
done 0x7ffccc93a5c0 0x56124b3a8eb0 5
My problem is:
For the 1st case, it is perfectly called by "move semantics", and as you see, the ownership of the vector has been transfered to the function parameter. Finally, at "done", the data is null to verify the the vector at main() no longer owns the vector.
Now to explicitly claim the parameter is "rvalue reference", as case 2. As you see, actually it is like "call by (l)reference".
How can I figure out it?
Tried to do it based on the official documentation. Here is the code, I'm trying to add a few items to an array:
auto in_array = bsoncxx::builder::stream::document{}
<< "$set" << open_document << call_id_key << call_id << close_document << "$push"
<< open_document << transcription_key
<< open_array << open_document << direction_key << "INBOUND" << text_key
<< transcript << confidence_key << confidence
<< start_time_key << start_time << end_time_key << end_time
<< "words" << open_array;
for (const auto& word_info : words) {
in_array = in_array << open_document << "word" << word_info.first <<
"confidence" << word_info.second << close_document; // Assertion happens here
}
bsoncxx::document::value doc = in_array << close_array << close_document << close_array << close_document << finalize;
std::cout << bsoncxx::to_json(doc.view()) << std::endl;
I am trying to get the valid memory address of CHAR A4 and b5 but when I try to reach that address using Hex Editor it's not reading the address that I have already got in my console output after compiling.
Hex Editor is validating the address as invalid address.
My Code:
#include <iostream>
using namespace std;
main ()
{
{ //INT
cout << "INT" << '\n';
int a = 2, b = 3;
cout << "Result: " << "for " << "int a " << "= " << a << '\n';
cout << "Result: " << "for " << "int a " << "= " << a << " " << "at " << "address " << &a << '\n';
cout << "Result: " << "for " << "int b " << "= " << b << '\n';
cout << "Result: " << "for " << "int b " << "= " << b << " " << "at " << "address " << &b << '\n';
cout << "-----------------------------------------" << '\n';
}
{
//SHORT
cout << "SHORT" << '\n';
short a = 2, b = 3;
cout << "Result: " << "for " << "short a " << "= " << a << '\n';
cout << "Result: " << "for " << "short a " << "= " << a << " " << "at " << "address " << &a << '\n';
cout << "Result: " << "for " << "short b " << "= " << b << '\n';
cout << "Result: " << "for " << "short b " << "= " << b << " " << "at " << "address " << &b << '\n';
cout << "-----------------------------------------" << '\n';
}
{
//FLOAT
cout << "FLOAT" << '\n';
float a = 2, b = 3.1;
cout << "Result: " << "for " << "float a " << "= " << a << '\n';
cout << "Result: " << "for " << "float a " << "= " << a << " " << "at " << "address " << &a << '\n';
cout << "Result: " << "for " << "float b " << "= " << b << '\n';
cout << "Result: " << "for " << "float b " << "= " << b << " " << "at " << "address " << &b << '\n';
cout << "-----------------------------------------" << '\n';
}
{
//DOUBLE
cout << "DOUBLE" << '\n';
double a = 20, b = 30.1;
cout << "Result: " << "for " << "double a " << "= " << a << '\n';
cout << "Result: " << "for " << "double a " << "= " << a << " " << "at " << "address " << &a << '\n';
cout << "Result: " << "for " << "double b " << "= " << b << '\n';
cout << "Result: " << "for " << "double b " << "= " << b << " " << "at " << "address " << &b << '\n';
cout << "-----------------------------------------" << '\n';
}
{
//CHAR
cout << "CHAR" << '\n';
char A4 = 'A' , b5 = 'B' ;
cout << "Result: " << "for " << "Char A4 " << "= " << A4 << '\n';
cout << "Result: " << "for " << "Char A4 " << "= " << A4 << " " << "at " << "address " << &A4 << '\n';
cout << "Result: " << "for " << "Char b5 " << "= " << b5 << '\n';
cout << "Result: " << "for " << "Char b5 " << "= " << b5 << " " << "at " << "address " << &b5 << '\n';
cout << "-----------------------------------------" << '\n';
}
}
Check out the list of overloads for operator<< for streams. The one for char const* assumes a zero-terminated string at that address. What you want is the overload for void const*. For other types of pointees, that conversion is done implicitly by the compiler, for char you need to make it explicitly yourself:
cout << static_cast<void const*>(&b5) << endl;
char A4 = 'A';
cout << &A4;
Is printing char*, when you print a char* the standard library tries to print a null terminated character string. As you only have a single character there is no null terminator so junk gets printed until the standard library happens to find a null byte, this is undefined behaviour.
To print a pointer rather than a string you need to cast to a different pointer type, for example:
char A4 = 'A';
cout << static_cast<void*>(&A4);
I have a problem with a school homework. I have to write a few function, and I have problem with the ShiftPointerVal which argument is a pointer to int. This is the most important in that homework, I mean that argument. I have that issue: error: lvalue required as left operand of assignment. Target of this function is to make a pointer bigger by one. I don't know how to write this function so the pointer to int can be an argument.
void ShiftPointerVal (int& pointer)
{
&pointer = &pointer + 1;
}
void ShiftPointerRef (int *&pointer)
{
pointer = pointer + 1;
}
void ShiftPointerPointer (int **pointer)
{
*pointer = *pointer +1;
}
int main ()
{
int number = 5;
int* p_number = &number;
std::cout << "\n----- 4 -----\n";
std::cout << &number << "\t" << p_number << "\n";
std::cout << *p_number << "\n";
ShiftPointerVal (p_number);
std::cout << &number << "\t" << p_number << "\n";
std::cout << *p_number << "\n";
std::cout << "\n----- 5 -----\n";
std::cout << &number << "\t" << p_number << "\n";
std::cout << *p_number << "\n";
ShiftPointerRef (*&p_number);
std::cout << &number << "\t" << p_number << "\n";
std::cout << *p_number << "\n";
std::cout << "\n----- 6 -----\n";
std::cout << &number << "\t" << p_number << "\n";
std::cout << *p_number << "\n";
ShiftPointerPointer (&p_number);
std::cout << &number << "\t" << p_number << "\n";
std::cout << *p_number << "\n";
}
I have a map declared as follows
map<string, int> symbolTable;
if(tempLine.substr(0,1) == "("){
symbolTable.insert(pair<string, int>(tempLine, lineCount));
}
How do I std::cout all of the things in my symbol table?
In modern C++:
for (auto&& item : symbolTable)
cout << item.first << ": " << item.second << '\n';
If you only have access to a pre-C++11 compiler the code would be:
for ( map<string, int>::const_iterator it = symbolTable.begin(); it != symbolTable.end(); ++it)
cout << it->first << ": " << it->second << '\n';
Here's an alternative if your compiler isn't C++11 compliant:
for (map<string, int>::iterator it = symbolTable.begin();
it != symbolTable.end(); ++it)
{
cout << it->first << " " << it->second << endl;
}
And for completeness, if it is:
for (auto& s : symbolTable)
{
cout << s.first << " " << s.second << endl;
}
You can use a loop to print all the key/value pairs. The code following is an example in C++11
for (const auto& kv : symbolTable) {
std::cout << kv.first << " " << kv.second << '\n';
}
ps: Both of other two answers pay little attention to const, which is quite sad...