How to covert a const char* [] to a single block of memory, say char* ptr

Question

const char* colors[] = {"Red", "Blue", "Green"};
*ptr = new char[30];
memset(ptr,0,30);

I want to store colors(2-D) to ptr(1-D), something like this =>

ptr = "Red";
ptr+10 = "Blue";
ptr+20 = "Green";

this gives me lvalue required as left operand of assignment

Could someone guide me with this, please?

[`strcpy`](https://en.cppreference.com/w/c/string/byte/strcpy) — Angew is no longer proud of SO, Jan 16 '19 at 14:26
Have you researched any possible solutions? What did you find? — Lightness Races in Orbit, Jan 16 '19 at 14:27
You could do `"Red\0Blue\0Green"` to get continuous memory containing those three strings, but this sounds like an XY problem. What are you *really* trying to do? — Blaze, Jan 16 '19 at 14:27
`std::ostringstream os; os << std::setw(10) << std::left << colors[0] << std::setw(10) << std::left << colors[1] << std::setw(10) << std::left << colors[2];` — Some programmer dude, Jan 16 '19 at 14:29
Second line is very strange. May be is char *ptr = new char[30]; ? — Mel Viso Martinez, Jan 16 '19 at 14:30
`ptr = "Red";` You've just re-assigned the pointer so the mem you `new`d is lost. — Johnny Mopp, Jan 16 '19 at 14:32
Whoever taught you that `char *` was an appropriate type for text was lying to you. Use `std::string`, it behaves how you want it here — Caleth, Jan 16 '19 at 14:37

bruno · Accepted Answer · 2019-01-16T14:47:28.240

0

you request that :

strcpy(ptr, "red");
strcpy(ptr+10, "Blue");
strcpy(ptr+20, "Green");

after it is your responsibility to know where you placed the strings

Can be more practical/generic to have only one null char between each and at least two null char after the last ?

But frankly there are more practical/safe solutions using std::vector ...

About the error "lvalue required as left operand of assignment", you have it for the lines

ptr+10 = "Blue";
ptr+20 = "Green";

ptr+n is not a lvalue but a value, typically it is not a variable

edited Jan 16 '19 at 14:47

answered Jan 16 '19 at 14:28

bruno

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please add some more explaination. Providing only the code is a good recipe to grow cargo cult programmers. Eg why does OPs code not work? – 463035818_is_not_a_number Jan 16 '19 at 14:36
Thanks!! it works!! I feel so silly asking this question :D Do you have any idea why I was getting that error with the way I was trying to do it? – Shubham Singla Jan 16 '19 at 14:36
@ShubhamSingla because you need to learn a little bit about C++ and built-in types and classes and... – Matthieu Brucher Jan 16 '19 at 14:42
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ptr+10 = "Blue"; you're trying to assign a value to an object which is not an lvalue. You can still do something like *(ptr+10) = "Blue"; but this will not do what you want – Gojita Jan 16 '19 at 14:44
@ShubhamSingla I edited my answer, saying the same as some other guys including Gojita above – bruno Jan 16 '19 at 14:49
"is not a lvalue but a value" -> " is not a lvalue but a rvalue" ? – 463035818_is_not_a_number Jan 16 '19 at 15:05
@user463035818 no, if I am not wrong a _rvalue_ (renamed _prvalue_) can be assigned (https://stackoverflow.com/questions/3601602/what-are-rvalues-lvalues-xvalues-glvalues-and-prvalues) – bruno Jan 16 '19 at 15:41
but "value" is just the most general term that does not imply that you cannot assign to it, no? – 463035818_is_not_a_number Jan 16 '19 at 15:42
@user463035818 in my personal a value cannot. What is the right name for you ? – bruno Jan 16 '19 at 15:44
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dont know, thats why I asked, my standardese is rather crappy, I always try to break things down to something simple that i can understand, which in this case is just "this wont work" but that doesnt qualify as an answer – 463035818_is_not_a_number Jan 16 '19 at 16:01

How to covert a const char* [] to a single block of memory, say char* ptr

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