Unable to concat strings in C++? - c++

I have a method to log with the following definition:
void log(std::string s) {
std::string tag = "main";
std::cout << tag << " :" << s << std::endl;
}
I'm trying to call this method like this:
log("direction" << std::to_string(direction) << ", count: " << std::to_string(count));
direction and count are integers.
I'm getting this following error with << underlined in red:
no operator << matches these operands.
operand types are const char [10] << std::string
I have #include<string> in my header to make sure my strings are working as they should.
I tried std::string("direction") and still the issue was same.
Beginner in C++. Help would be appreciated.

operator<< isn't used for arbitrary string concatenation - it is called an "output stream operator", and it is only used in the context of std::ostream.
When you say...
std::cout << tag << " :" << s << std::endl;
...you're actually writing code roughly equivalent to:
std::cout.operator<<(tag).operator<<(" :").operator<<(s).operator<<(std::endl);
As you can see operator<< knows how to work with std::cout and std::string, but not between strings.
In order to concatenate std::string instances, you can simply use operator+:
log("direction" + std::to_string(direction) + ", count: " + std::to_string(count));
Please note that this concatenation technique is not the most efficient: you might want to look into std::stringstream or simply use std::string::reserve to avoid unnecessary memory allocations.

Substitute the << with the + operator as you are manipulating the string, not the stream:
log("direction" + std::to_string(direction) + ", count: " + std::to_string(count));

If you're determined to use the operator<< notation you need an object that understands it.
Here's such an object (I make no claims that this is a good idea):
#include <string>
#include <sstream>
#include <iostream>
void log(std::string s) {
std::string tag = "main";
std::cout << tag << " :" << s << std::endl;
}
struct string_accumulator
{
std::ostringstream ss;
template<class T>
friend string_accumulator& operator<<(string_accumulator& sa, T const& value)
{
sa.ss << value;
return sa;
}
template<class T>
friend string_accumulator& operator<<(string_accumulator&& sa, T const& value)
{
return operator<<(sa, value);
}
operator std::string () { return ss.str(); }
};
inline auto collect() -> string_accumulator
{
return string_accumulator();
}
int main()
{
int direction = 1;
int count = 1;
log(collect() << "direction" << std::to_string(direction) << ", count: " << std::to_string(count));
}

The prototype of your function is void log(std::string s);. It awaits for an std::string. So you need to pass a string to it, not a stream!
So, change this:
log("direction" << std::to_string(direction) << ", count: " << std::to_string(count));
to this:
log("direction" + std::to_string(direction) + ", count: " + std::to_string(count));
where I only changed the << operator to + operator. It will now concatenate everything inside the parentheses to a single std::string.
Your attempt implies that you wanted to pass std::ostream as the parameter. Maybe you want to read C++ Passing ostream as parameter. However, if I were you, I would just overload <<.

why don't you use:
// just include thisusing namespace std;

Related

Simpler logger function using std::stringstream

I'm trying to write a simple logging function which can handle different types in this format:
LOG("This is one type: " << one_type << " and here is a different type: " << diff_type);
I've been looking at the examples here:
How to use my logging class like a std C++ stream?
stringstream with recursive variadic function?
and this is what I came up with:
#include <iostream>
#include <sstream>
void LOG(std::stringstream& ss)
{
std::cout << ss.str() << std::endl;
}
However, even when I just do:
LOG("Some text");
I get the error:
could not convert ‘(const char*)"Some text"’ from ‘const char*’ to ‘std::stringstream’ {aka ‘std::__cxx11::basic_stringstream<char>’}
How can I implement this?
A common way to solve this is to make LOG a macro that just does text substitution instead. You could define a LOG macro like
#define LOG(to_log) \
do \
{ \
std::cout << to_log << std::endl; \
} while (false)
and then
LOG("This is one type: " << one_type << " and here is a different type: " << diff_type);
would get expanded to
do
{
std::cout << "This is one type: " << one_type << " and here is a different type: " << diff_type << std::endl;
} while (false);
template <typename ... T>
void log(T const& ... t)
{
( std::cout << ... << t );
// maybe want to add a newline?
std::cout << std::endl; // note: '\n' just adds it,
// std::endl flushes the stream...
}
would allow to log (to console!) as:
log("this is an int: ", 1210, " and this is a double: ", 10.12);
Note the commas instead of the stream operator (<<)...
If you want to log to arbitrary streams you might need to add it as yet another parameter (/*...*/ void log(std::ostream& s, T const&... t)) or you might (among other solutions) initialise some singleton that you access from within the logging function:
void initialiseLogging(std::ostream& s)
{
Logger::instance().initialize(s);
}
template <typename ... T>
void log(T const& ... t)
{
// assumes returning a reference:
( Logger::instance().stream() << ... << t );
}
Above variant assumes initialisation being required and logging without yielding undefined behaviour (needs to be documented!). If you want to be able to log without you need to change:
{
// now assuming a pointer being returned!
auto s = Logger::instance().stream();
// variant 1: log to nowhere if not initialised:
if(s)
{
( *s << ... << t );
}
// variant 2: default logging to console
s = s ? s : &std::cout;
( *s << ... << t );
}

Overload << operator to change " " to "\n"

I am trying to overload
<<
operator. For instance
cout << a << " " << b << " "; // I am not allowed to change this line
is given I have to print it in format
<literal_valueof_a><"\n>
<literal_valueof_b><"\n">
<"\n">
I tried to overload << operator giving string as argument but it is not working. So I guess literal
" "
is not a string. If it is not then what is it. And how to overload it?
Kindly help;
Full code
//Begin Program
// Begin -> Non - Editable
#include <iostream>
#include <string>
using namespace std;
// End -> Non -Editable
//---------------------------------------------------------------------
// Begin -> Editable (I have written )
ostream& operator << (ostream& os, const string& str) {
string s = " ";
if(str == " ") {
os << '\n';
}
else {
for(int i = 0; i < str.length(); ++i)
os << str[i];
}
return os;
}
// End -> Editable
//--------------------------------------------------------------------------
// Begin -> No-Editable
int main() {
int a, b;
double s, t;
string mr, ms;
cin >> a >> b >> s >> t ;
cin >> mr >> ms ;
cout << a << " " << b << " " ;
cout << s << " " << t << " " ;
cout << mr << " " << ms ;
return 0;
}
// End -> Non-Editable
//End Program
Inputs and outputs
Input
30 20 5.6 2.3 hello world
Output
30
20
5.6
2.3
hello
world
" " is a string-literal of length one, and thus has type const char[2]. std::string is not related.
Theoretically, you could thus overload it as:
auto& operator<<(std::ostream& os, const char (&s)[2]) {
return os << (*s == ' ' && !s[1] ? +"\n" : +s);
}
While that trumps all the other overloads, now things get really hairy. The problem is that some_ostream << " " is likely not uncommon, even in templates, and now no longer resolves to calling the standard function. Those templates now have a different definition in the affected translation-units than in non-affected ones, thus violating the one-definition-rule.
What you should do, is not try to apply a global solution to a very local problem:
Preferably, modify your code currently streaming the space-character.
Alternatively, write your own stream-buffer which translates it as you wish, into newline.
Sure this is possible, as I have tested. It should be portable since you are specifying an override of a templated function operator<<() included from <iostream>. The " " string in your code is not a std::string, but rather a C-style string (i.e. a const char *). The following definition works correctly:
ostream& operator << (ostream& os, const char *str) {
if(strcmp(str, " ") == 0) {
os << '\n';
} else {
// Call the standard library implementation
operator<< < std::char_traits<char> > (os, str);
}
return os;
}
Note that the space after std::char_traits<char> is necessary only if you are pre-c++11.
Edit 1
I agree with Deduplicator that this is a potentially dangerous solution as it may cause undesirable consequences elsewhere in the code base. If it is needed only in the current file, you could make it a static function (by putting it within an unnamed namespace). Perhaps if you shared more about the specifics of your problem, we could come up with a cleaner solution for you.
You might want to go with a user defined literal, e.g.
struct NewLine {};
std::ostream& operator << (std::ostream& os, NewLine)
{
return os << "\n";
}
NewLine operator ""_nl(const char*, std::size_t) // "nl" for newline
{
return {};
}
This can be used as follows.
int main(int, char **)
{
std::cout << 42 << ""_nl << "43" << ""_nl;
return 0;
}
Note three things here:
You can pass any string literal followed by the literal identifier, ""_nl does the same thing as " "_nl or "hello, world"_nl. You can change this by adjusting the function returning the NewLine object.
This solution is more of an awkward and confusing hack. The only real use case I can imagine is pertaining the option to easily change the behavior at a later point in time.
When doing something non-standard, it's best to make that obvious and explicit - here, the user defined literal indeed shines, because << ""_nl is more likely to catch readers' attention than << " ".

How do I stream into a string without creating a named stringstream?

I often end up writing code like this:
SomeStreamableType x;
std::stringstream ss;
ss << "Value is: " << x;
log(ss.str());
The extra line needed to generate the stringstream feels superfulous. I can do this, but it's equally cumbersom:
SomeStreamableType x;
const std::string str = "Value is: " + boost::lexical_cast<std::string>(x);
log(str);
I want to be able to do this:
SomeStreamableType x;
log(std::stringstream() << "Value is: " << x);
Have others encountered this issue and come up with a workaround? I don't want to create any helper functions or classes.
Your code will work without modifications, as long as log accepts an ostream& reference:
void log(ostream& o) {
stringstream* s = dynamic_cast<stringstream*>(&o);
if (s) {
cout << s->str() << endl;
}
}
int main() {
int x = 5, y = 6;
log(stringstream() << "x=" << x << ", y=" << y);
return 0;
}
Demo.
To solve this problem I have often simply done something like this:
#define LOG(m) do{std::ostringstream oss;oss<<m;std::cout<<oss.str()<<'\n';}while(0)
// ...
LOG("some text: " << value1 << ' ' << value2); // no need for '\n'
Now I tend to use a more sophisticated class based solution that has an even nicer interface and doesn't use a horrible macro.

How to print a bunch of integers with the same formatting?

I would like to print a bunch of integers on 2 fields with '0' as fill character. I can do it but it leads to code duplication. How should I change the code so that the code duplication can be factored out?
#include <ctime>
#include <sstream>
#include <iomanip>
#include <iostream>
using namespace std;
string timestamp() {
time_t now = time(0);
tm t = *localtime(&now);
ostringstream ss;
t.tm_mday = 9; // cheat a little to test it
t.tm_hour = 8;
ss << (t.tm_year+1900)
<< setw(2) << setfill('0') << (t.tm_mon+1) // Code duplication
<< setw(2) << setfill('0') << t.tm_mday
<< setw(2) << setfill('0') << t.tm_hour
<< setw(2) << setfill('0') << t.tm_min
<< setw(2) << setfill('0') << t.tm_sec;
return ss.str();
}
int main() {
cout << timestamp() << endl;
return 0;
}
I have tried
std::ostream& operator<<(std::ostream& s, int i) {
return s << std::setw(2) << std::setfill('0') << i;
}
but it did not work, the operator<< calls are ambigous.
EDIT I got 4 awesome answers and I picked the one that is perhaps the simplest and the most generic one (that is, doesn't assume that we are dealing with timestamps). For the actual problem, I will probably use std::put_time or strftime though.
In C++20 you'll be able to do this with std::format in a less verbose way:
ss << std::format("{}{:02}{:02}{:02}{:02}{:02}",
t.tm_year + 1900, t.tm_mon + 1, t.tm_mday,
t.tm_hour, t.tm_min, t.tm_sec);
and it's even easier with the {fmt} library that supports tm formatting directly:
auto s = fmt::format("{:%Y%m%d%H%M%S}", t);
You need a proxy for your string stream like this:
struct stream{
std::ostringstream ss;
stream& operator<<(int i){
ss << std::setw(2) << std::setfill('0') << i;
return *this; // See Note below
}
};
Then your formatting code will just be this:
stream ss;
ss << (t.tm_year+1900)
<< (t.tm_mon+1)
<< t.tm_mday
<< t.tm_hour
<< t.tm_min
<< t.tm_sec;
return ss.ss.str();
ps. Note the general format of my stream::operator<<() which does its work first, then returns something.
The "obvious" solution is to use a manipulator to install a custom std::num_put<char> facet which just formats ints as desired.
The above statement may be a bit cryptic although it entirely describes the solution. Below is the code to actually implement the logic. The first ingredient is a special std::num_put<char> facet which is just a class derived from std::num_put<char> and overriding one of its virtual functions. The used facet is a filtering facet which looks at a flag stored with the stream (using iword()) to determine whether it should change the behavior or not. Here is the code:
class num_put
: public std::num_put<char>
{
std::locale loc_;
static int index() {
static int rc(std::ios_base::xalloc());
return rc;
}
friend std::ostream& twodigits(std::ostream&);
friend std::ostream& notwodigits(std::ostream&);
public:
num_put(std::locale loc): loc_(loc) {}
iter_type do_put(iter_type to, std::ios_base& fmt,
char fill, long value) const {
if (fmt.iword(index())) {
fmt.width(2);
return std::use_facet<std::num_put<char> >(this->loc_)
.put(to, fmt, '0', value);
}
else {
return std::use_facet<std::num_put<char> >(this->loc_)
.put(to, fmt, fill, value);
}
}
};
The main part is the do_put() member function which decides how the value needs to be formatted: If the flag in fmt.iword(index()) is non-zero, it sets the width to 2 and calls the formatting function with a fill character of 0. The width is going to be reset anyway and the fill character doesn't get stored with the stream, i.e., there is no need for any clean-up.
Normally, the code would probably live in a separate translation unit and it wouldn't be declared in a header. The only functions really declared in a header would be twodigits() and notwodigits() which are made friends in this case to provide access to the index() member function. The index() member function just allocates an index usable with std::ios_base::iword() when called the time and it then just returns this index. The manipulators twodigits() and notwodigits() primarily set this index. If the num_put facet isn't installed for the stream twodigits() also installs the facet:
std::ostream& twodigits(std::ostream& out)
{
if (!dynamic_cast<num_put const*>(
&std::use_facet<std::num_put<char> >(out.getloc()))) {
out.imbue(std::locale(out.getloc(), new num_put(out.getloc())));
}
out.iword(num_put::index()) = true;
return out;
}
std::ostream& notwodigits(std::ostream& out)
{
out.iword(num_put::index()) = false;
return out;
}
The twodigits() manipulator allocates the num_put facet using new num_put(out.getloc()). It doesn't require any clean-up because installing a facet in a std::locale object does the necessary clean-up. The original std::locale of the stream is accessed using out.getloc(). It is changed by the facet. In theory the notwodigits could restore the original std::locale instead of using a flag. However, imbue() can be a relatively expensive operation and using a flag should be a lot cheaper. Of course, if there are lots of similar formatting flags, things may become different...
To demonstrate the use of the manipulators there is a simple test program below. It sets up the formatting flag twodigits twice to verify that facet is only created once (it would be a bit silly to create a chain of std::locales to pass through the formatting:
int main()
{
std::cout << "some-int='" << 1 << "' "
<< twodigits << '\n'
<< "two-digits1='" << 1 << "' "
<< "two-digits2='" << 2 << "' "
<< "two-digits3='" << 3 << "' "
<< notwodigits << '\n'
<< "some-int='" << 1 << "' "
<< twodigits << '\n'
<< "two-digits4='" << 4 << "' "
<< '\n';
}
Besides formatting integers with std::setw / std::setfill or ios_base::width / basic_ios::fill, if you want to format a date/time object you may want to consider using std::put_time / std::gettime
For convenient output formatting you may use boost::format() with sprintf-like formatting options:
#include <boost/format.hpp>
#include <iostream>
int main() {
int i1 = 1, i2 = 10, i3 = 100;
std::cout << boost::format("%03i %03i %03i\n") % i1 % i2 % i3;
// output is: 001 010 100
}
Little code duplication, additional implementation effort is marginal.
If all you want to do is output formatting of your timestamp, you should obviously use strftime(). That's what it's made for:
#include <ctime>
#include <iostream>
std::string timestamp() {
char buf[20];
const char fmt[] = "%Y%m%d%H%M%S";
time_t now = time(0);
strftime(buf, sizeof(buf), fmt, localtime(&now));
return buf;
}
int main() {
std::cout << timestamp() << std::endl;
}
operator<<(std::ostream& s, int i) is "ambiguous" because such a function already exists.
All you need to do is give that function a signature that doesn't conflict.

print time on each call to std::cout

How would someone do that?
for example I do like:
std::cout << "something";
then it should print the time before "something"
Make your own stream for that :) This should work:
class TimedStream {
public:
template<typename T>
TimedStream& operator<<(const T& t) {
std::cout << getSomeFormattedTimeAsString() << t << std::endl;
return *this;
}
};
TimedStream timed_cout;
void func() {
timed_cout << "123";
}
You'd be able to use this class for every type for which std::cout << obj can be done, so no further work is needed.
But please note that the time will be written before every <<, so you cannot chain them easily. Another solution with explicit timestamp is:
class TimestampDummy {} timestamp;
ostream& operator<<(ostream& o, TimestampDummy& t) {
o << yourFancyFormattedTimestamp();
}
void func() {
cout << timestamp << "123 " << 456 << endl;
}
You could use a simple function that prints the timestamp and then returns the stream for further printing:
std::ostream& tcout() {
// Todo: get a timestamp in the desired format
return std::cout << timestamp << ": ";
}
You would then call this function instead of using std::cout directly, whenever you want a timestamp inserted:
tcout() << "Hello" << std::endl;
ostream& printTimeWithString(ostream& out, const string& value)
{
out << currentTime() << ' ' << value << std::endl;
return out;
}
Generate current time using your favourite Boost.DateTime output format.
This looks like homework. You want something in the line of:
std::cout << time << "something";
Find a way the retrieve the time on your system, using a system call.
Then you'll have to implement a << operator for your system-dependent time class/struct.