用C / C ++连接字符串

Question

How do I concatenate Strings with C/C++?

I tried the following ways:

PS: errorInfo is a char * I should return it.

        errorInfo = strcat("Workflow: ", strcat(
            workflowToString(workflow).utf8(), strcat(" ERROR: ",
                    errorCode.utf8)));

        sprintf(errorInfo, "Workflow %s ERROR: %s",
            workflowToString(workflow).utf8(), errorCode.utf8());

        errorInfo = "Workflow: " + workflowToString(workflow).utf8() + " ERROR: " + errorCode.utf8;

Just the sprintf compiles but when running my application crash.

PS: I'm using NDK from Android

Answer 1

There ISN'T such a language as C/C++. There is C, and there is C++.

• In C++ you concatenate std::string 's by using operator+
• In C, you use strcat

I know this doesn't quite answer your question, this is just an outcry :)

Answer 2

According to this page strcat does the following:

Appends a copy of the source string to the destination string. The terminating null character in destination is overwritten by the first character of source, and a new null-character is appended at the end of the new string formed by the concatenation of both in destination.

In your implementation, however, "Workflow: " is a constant string. You cannot modify that string, which is what strcat would do. In order to do that, create a string like:

char message[1000];
strcpy(message, "Workflow: ");
strcat(message, "other string");
....

However, be careful about the utf8 character encoding because one utf8 code point could be multiple char s long.

Answer 3

Concatenation is almost always the wrong idiom for string building, especially in C. It's error-prone, clutters your code, and has extremely bad asymptotic performance (ie O(n^2) instead of O(n) for building a string of length n ).

Instead you should use the snprintf function, as in:

snprintf(buf, sizeof buf, "Workflow: %s ERROR: %s", workflow, error);

or if you're writing to a file/socket/etc. and don't need to keep the resulting string in memory, simply use fprintf to begin with.

Answer 4

There are many ways you can concatenate in C while using Android NDK:

Two ways I used are:

• strcat
• sprintf

here is example:

enter code here

strcat

char* buffer1=(char*)malloc(250000);
char* buffer2=(char*)malloc(250000);
char* buffer3=(char*)malloc(250000);

buffer1 = strcat(buffer1, buffer2);

sprintf

sprintf(buffer3,"this is buffer1: %s and this is buffer2:%s",buffer1,buffer2);`

sprintf returns length of your string

strcat is not recommended as its use more memory.. you can use sprintf or others like strcpy.

Hope it helps.

Answer 5

By using strcat() , you are working in c, not c++. c is not going to automatically manage memory for you. c can be confusing since sometimes it seems like it has a string data type when all it is doing is providing you a string interface to arrays of characters. For one thing, the first argument to strcat() has to be writable and have enough room to add the second string.

char *out = strcat("This", "nThat");

is asking c to stomp on string literal memory.

In general, you should NEVER use strcat()/sprintf , as in the above "chosen" answer. You can overwrite memory that way. Use strncat()/snprintf() instead to avoid buffer overruns. If you don't know the size to pass to "n" in strncat() , you're likely doing something wrong.

One way to do this in c would be:

 #define ERROR_BUF_SIZE  2048  // or something big enough, you have to know in c




char errorInfo[ERROR_BUF_SIZE];

   snprintf(errorInfo, ERROR_BUF_SIZE, "Workflow %s ERROR: %s",
            workflowToString(workflow).utf8(), errorCode.utf8());

or similarly using strncpy/strncat

Answer 6

With string literals you can simple use:

char str[] = "foo" " bar";
const char *s = " 1 " " 2 ";
s = " 3 " " 4 ";