Assign or construct a std::string given multiple char* strings

Question

When initializing a std::string like this:

std::string { "hello" + " c++ " + " world" }; // error

It errors because const char* cannot be added.

Meanwhile:

std::string { "hello" " c++ " " world" }; // ok

It works thanks to compiler magic.

And then, according to Google's coding-guide, I use constexpr since statements should be not be hard coded:

constexpr const char* HELLO = "hello"; constexpr const char* LANG_CPP = " c++"; constexpr const char* WORLD = " world";

Now things are different:

std::string str { HELLO LANG_CPP WORLD }; // error

Implicit concatenation doesn't work anymore.

Eventually, I wrote code like this:

std::string str = HELLO; str += LANG_CPP; str += WORLD;

Other options like below are not appealing to me:

std::string str = std::string(HELLO) + std::string(LANG_CPP) + std::string(WORLD);

std::string str = HELLO + std::string(LANG_CPP) + WORLD;

Do you have alternatives that are better looking?

Update: I have simplified the code. In order to focus on the matter I am concerned with.

My original code looked like this:

sql = "delete from " TABLE_NAME ";";

sql = "insert into " TABLE_NAME "values(" var1 ", " var2 ");";

By the way, the result of concatenation is not required to be a string, const char* is ok, too.

john · Accepted Answer · 2020-05-02 05:12:12Z

3

If you are using C++14 or later you can write

sql = "delete from "s + TABLE_NAME + ";"s;

answered May 2, 2020 at 5:12

john

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4 Comments

kayjay Over a year ago

first of all thx for your quick advice. if i want to use string literal then implicit concatenation just fit. the thing is to use (constexpr) const char *

john Over a year ago

You want TABLE_NAME to be a constexpr const char*? The above code works in that case.

kayjay Over a year ago

"delete from " is also hard coded in this case.. i think it need to be replaced like constexpr const char * DELETE_COMMAND = "delete from ";

Remy Lebeau Over a year ago

Note that you need using namespace std::string_literals; to use the ""s operator.

Remy Lebeau · Accepted Answer · 2020-05-02 06:00:10Z

The only way to make this work:

std::string str { HELLO LANG_CPP WORLD };

Is to use preprocessor #define macros instead of constexpr variables, eg:

#define HELLO "hello" #define LANG_CPP " c++" #define WORLD " world"

So that the compiler sees this:

std::string { "hello" " c++ " " world" };

You have already covered most other uses of const char*/std::string concatenations, but one you didn't mention is std::ostringstream, eg:

#include <sstream> constexpr const char* HELLO = "hello"; constexpr const char* LANG_CPP = " c++"; constexpr const char* WORLD = " world"; std::ostringstream oss; oss << HELLO << LANG_CPP << WORLD; std::string str = oss.str();

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