![]() ![]() This example is similar to the Mandelbrot example code in the Qt documentation, however the code presented here is much simpler and easier to follow. ![]() Here we consider the situation when a GUI-driven application needs to generate a QImage, however, due to the time required to generate the QImage, the GUI would freeze unless the QImage is generated in a separate thread. Lines not including the prefix will be completely skipped. MySQL will skip the prefix, and anything that comes before it. For example, if each line with real data begins with DATA_ENTRY, and you want to ignore all others, add: MySQL can read only the lines in a data file containing a specified prefix if necessary. The columns that each piece of data should be inserted into are also specified. Here we have specified that the data is comma-delimited, and each line of the input file corresponds to a row in the table. (field1, field2, field3, field4, field5) Note that if you don't include LOCAL, the file you want to import must be located on the server, rather than a client.įor example, suppose you have an ASCII file containing comma-delimited data that you want to insert into the table my_table. Rather than using a Perl script, for example, data can be imported directly into MySQL using LOAD DATA LOCAL INFILE. Here a double is always returned by the function, but it has two input arguments, which can be of different types. This of course can be trivially extended to multiple data types, for example:ĭouble SumTwoArrays(T *array1, U *array2, int num1, This allows you to determine a function with a generic data type. The most obvious way to do this is to use overloading, and explicitly define the required functions, all with the same name and content except for the data type.Īn alternative is to use a template. Have you ever needed to have multiple versions of the same function, with the only difference being the date type used for the arguments and/or value returned? For example, you might need versions of the same function that work with int, float and double data types. The thing here that confuses some people is that the type must be specified here as well. Any instance of the class myclass has the same value of i. There is only a single variable called i. Then c1 and c2 will contain their own independent variable i. If we then create two objects of this class: Then other files can access the variable pi provided they contain:ģ. Other files are not able to access the variable pi. Then the global variable pi can be used inside just this one file. Here, the variable count is set to zero only once, not 1000 times. The variable count will be set to zero only once, even if the function is called millions of times. If a function contains the following line: If we want to ensure that it cannot be modified, we would make use of const:ġ. This of course means that it is possible to modify this variable. Which means that inside the function we don't just have a copy of the mystructure variable, we have variable itself. Instead, we could pass the structure in by reference: This may take a long time, and/or may use too much memory. When myfunction is used, a copy of the structure is made which is available only inside the function. For example, firstly consider:Īssume mystructure uses a large amount of memory. In this case, we cannot change the value of x using the pointer px, and we cannot point px to another address.Īnother usage is in passing parameters to functions. The value x can be changed, however the address pointed to by px can't be changed. The difference here is that the const is after the *. The important thing to remember is that while *px can be pointed to any integer, you cannot change the value of the integer. This basically means that *px is a const int, i.e. Clearly you must initialize constant variables when they are declared, as you can't change their value later! Using const in this context is an alternative to using #define, for example: The value of the variable i cannot be changed. This just means that the variable i is always 8 and cannot be changed. The simplest usage of const is this, for example: The const keyword allows you to specify whether a particular variable can be modified or not. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |