c++ | 易学教程

Read cells from Excel in C++?

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问题 I was wondering how you can read specific cells from an Excel spreadsheet, in C++. I understand we have to use the "fstream" library, but I don't know exactly how I could get those values from a certain cell, and print it on the screen. Any help would be appreciated, thanks! Carpetfizz 回答1: in linux you have this free: http://libxls.sourceforge.net/ in windows you have http://www.libxl.com/ which seems to cost money: Book* book = xlCreateBook(); if(book) { if(book->load(L"example.xls")) {

gcc doesn't accept out-of-line definition of member with non-type template parameter defined via nested templated using clause

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问题 The title seems convoluted but our test-case is actually a minimal example for a real case. We have code for which we want to choose implementation of a method based on the template parameter. We during clean up we defined conditional enable_if_t with using clause, and as next step wanted to put definition out of line, this produced following code: #include <type_traits> #include <iostream> #include <string> template <typename T> struct A { template <typename U> using is_int_or_float = std:

Eliminating instantiation of useless destructor calls?

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问题 Well, my colleague is pretty in depth nitpicking about eliminating unnecessarily code instantiations for destructor functions. Still same situation, as mentioned in this question: Very limited space for .text section (less 256 KB) Code base should scale among several targets, including the most limited ones Well known use cases of the code base by means some destructor logic is neccesary to manage object lifetimes or not (for many cases life-time of objects is infinite, unless the hardware is

Python的IDE之Pycharm的使用

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Python的IDE之Pycharm的使用一、为什么用IDE(Python集成开发环境-Pycharm) 到现在为止，我们也是写过代码的人啦，但你有没有发现，每次写代码要新建文件、写完保存时还要选择存放地点，执行时还要切换到命令行调用python解释器，好麻烦呀，能否一气呵成，让我简单的写代码？此时开发工具IDE上场啦，一个好的IDE能帮你大大提升开发效率。很多语言都有比较流行的开发工具，比如JAVA 的Eclipse， C#，C++的VisualStudio， Python的是啥呢？ Pycharm和Jupyter，最好的两款Python开发IDE。二、安装Pycharm(老汉推车式) 接下来安装pycharm 1、首先从网站下载pycharm：点击打开链接（链接为： http://www.jetbrains.com/pycharm/download/#section=windows），进入之后如下图，根据自己电脑的操作系统进行选择，对于windows系统选择图中红色圈中的区域。 2、在弹出的PyCharm安装欢迎页面中，点击“Next按钮”进入下一步 3、根据你电脑的处理器选择32位或64位，创建桌面快捷方式、关联*.py文件 4、选择开始菜单文件夹，选择Install 5、开始安装 6、安装完成，立刻运行Pycharm 7、选择是否导入开发环境配置文件

Copy-on-write file mapping on windows

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问题 I have 3 processes communicating over named pipes: server, writer, reader. The basic idea is that the writer can store huge (~GB) binary blobs on the server , and the reader(s) can retrieve it. But instead of sending data on the named pipe, memory mapping is used. The server creates an unnamed file-backed mapping with CreateFileMapping with PAGE_READWRITE protection, then duplicates the handle into the writer . After the writer has done its job, the handle is duplicated into any number of

Set a StyleSheet for a whole widget in Qt

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问题 I have a custom widget which inherits from QWidget and contains some labels in its layout. I would like to change the background color of the widget and the labels in the widget (this is, everything!) every time I put the mouse over it. When using *:hover { background: red; } in my custom widget, I only get the contents red when moving the mouse over the labels, but not outside them, between labels, etc. I don't understand this behavior taking into account that I put the StyleSheet in the

WndProc calling mechanism (WinAPI)

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问题 I'm trying to understand how a windows application works. There is a WndProc function, wherein message processing occurs. LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam) { switch (msg) { case WM_KEYDOWN: if (wParam == VK_ESCAPE) { if (MessageBox(0, L"Are you sure?", L"Exit?", MB_YESNO | MB_ICONQUESTION) == IDYES) //Release the windows allocated memory DestroyWindow(hwnd); } return 0; case WM_DESTROY: PostQuitMessage(0); return 0; } return DefWindowProc(hwnd, msg,

How do I declare a vector of vector::size_type?

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问题 I want a vector whose elements are of the type vector::size_type However, you cannot declare: vector<vector::size_type> aVec; because size_type is part of the template itself, so you have to use the type itself, I need something like: vector<vector<T>::size_type> aVec; but what should the T be? It's really a circular problem. :) If vector had just used size_t as its size type (but not had a special typedef for the size_type which could vary maybe based on the type that the vector is holding),

How do I declare a vector of vector::size_type?

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Casting from member pointer to whole struct/class

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问题 Consider following code: #include <iostream> struct bar { double a = 1.0; int b = 2; float c = 3.0; }; void callbackFunction(int* i) { auto myStruct = reinterpret_cast<bar*>(i) - offsetof(bar, b); std::cout << myStruct->a << std::endl; std::cout << myStruct->b << std::endl; std::cout << myStruct->c << std::endl; //do stuff } int main() { bar foo; callbackFunction(&foo.b); return 0; } I have to define a callback function and I want to use some additional information in that function. I defined